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METAMORPHISM

ROCKS

04
CM
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. A, IRVING

REESE LIBRARY

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UNIVERSITY OF CALIFORNIA.

^Received

^Accession No. /$'<, k> . Cljss No.

METAMORPHISM OF ROCKS.

CHEMICAL AND PHYSICAL STUDIES

IN THE

METAMORPHISM OF ROCKS

BASED ON

A THESIS (WITH APPENDICES) WRITTEN FOB THE
DOCTORATE IN SCIENCE IN THE

UNIVERSITY OP LONDON.

A. IEVING, D.Sc., B.A., F.G.S.

LONDON

LONGMANS, GREEN, AND Co.

AND NEW YORK : 15 EAST 16 th STREET

1889

QE41-S
-7

TO THB

REV. T. G. BONNET, D.So., LL.D., F.R.S.,

Past President of the Geological Society ,
Professor of Geology in University College, London ;
Fellow of St. John's College, Cambridge ;
Honorary Canon of Manchester ;

in admiration of his skill in microscopic petrology and as a field -geologist, and
of his extensive culture ; this little work is dedicated by

THE AUTHOE.

PREFATORY NOTE.

The inception of this little work is due to the Presidential
Address of Professor Bonney to the Geological Society in
1886. When that Address appeared it seemed to me that
some of the leading ideas contained in it would admit of a
fuller consideration from the chemical and physical side ; and
in this I was happy to find that the author of the Address
concurred. My first attempt to deal with them was in a
paper which I hastily put together for the Birmingham
Meeting (1886) of the British Association. I found however
that even in its incipient stage the subject was too vast to be
dealt with satisfactorily in a paper, and I had to content
myself with a brief statement of some of the leading points,
which appeared in the Association's Eeport for that year.

In writing the Thesis on Bock-Metamorphism I was fully
conscious of many imperfections in the treatment of some
portions of the subject. It was especially so with parts of
Sections ii and iii, which were written for the most part in
1886. As the other parts and the subsidiary matters con-
tained in Appendix ii grew to considerable proportions, I
found that with my daily work and the inroad which two
other papers* made upon my time in 1887, I was not able to
re-cast Sections ii and iii as I could have wished to do,
without risking the delay of another whole year in sending in
the Thesis, and this for obvious reasons it would have been
unwise to do. These matters were worked out more fully in
a supplement, copies of which have been privately distributed
along with the Thesis. The Thesis has been submitted to
some of the highest authorities in this country and on the
Continent ; and the friendly acknowledgements it has met
with abroad have been to me encouraging in the highest
degree. The matter contained in the Supplement is now
published at the suggestion of the University Examiners
incorporated with the original Thesis, which has undergone
careful revision, the alterations being however to a great
extent merely verbal. Some few further additions have been
made both to the body of the work and to the original
appendices ; and these together with the matter contained in

* See Q.J.G.S. for May, 1888.

VU1. PREFATORY NOTE.

the Supplement have been printed in smaller type. A little
delay in the publication has enabled me to draw attention
here and there to valuable contributions to petrology made by
foreign geologists of eminence in the ' Etudes sur les Schistes
Crystallins ' published by the International Geological Con-
gress, which met in London, in September, 1888.

It would not be possible, were I to attempt it, to express
my indebtedness to Professor Hermann Credner of Leipzig,
whose masterly and philosophical work, ' Elemente der
Geologie,' as it stands in the sixth edition, (1887), is still
without a rival in our language, as a storehouse of geological
facts and principles. * It is some gratification to me to
find my own conclusions on some of the more important
points in connexion with the genesis of the crystalline rocks
so thoroughly in accord with those of Thomas Macfarlane,
Esq., F.E.S.C., the result in his case of very extensive
experience both as a metallurgist and as a field-geologist.
It was with the greatest pleasure that I read his ' Origin of
the Eruptive and Primary Rocks ' (written in 1864) after the
earlier sheets of this work were printed off, the author
having kindly given me a copy of it at the International
Congress in London.

The departure which our University has now made in
recognising ' original work ' and in doing its part in lifting the
discussion of higher scientific questions out of the professional
arena is of itself the best reply to the criticisms that have
been passed in some quarters, and augurs well for the future of
Science in this country. It may be questioned whether the
outcry referred to is not to a great extent a reflex of a
belief in that system of over-teaching which has done so
much to check the growth of original scientific thought in this
country, as compared with some of the leading nations of
the civilized world, by giving too much advantage in the
academical race to mere receptivity. It seems to be forgotten
at times that the truest ' teaching ' is that which stimulates
the mind to active thought, not that which saves the student
the trouble of thinking by loading the memory with second-
hand knowledge. One necessary consequence of this is seen
in the rareness of appeals in a great deal of the scientific
literature of this country directly to nature, as compared
with the quotation of names of authority as giving weight
to certain ' views/ There are reasons for doubting whether,
with the exception. of the Royal Society (which represents

* Where not otherwise indicated the references to it in this work are to the
3rd Leipzig edition.

PEEFATOEY NOTE. IX.

all the sciences) there is a single scientific society in London
which is entirely free from the interested influence of a
close profession. In so far as such an influence is allowed
to exert itself there grows up a tendency to fetter the
discussion of scientific questions by a spurious ' orthodoxy ' ;
and just so far does such a society come short of the
fulfilment of its highest function, which is the advancement
of pure science, in the sense of an extension and correction
of our knowledge of the material universe.

Truth is indeed ' a pearl of great price,' and not easy to
find amid the heap of ' wood, hay, and stubble ' of that
department of literature which calls itself 'geological.' It is
110 difficult task for one possessed of a fair amount of training
in the literary art to write on natural subjects so as to appear
very profound to those who know a little less than he does.
Those on the other hand to whom it has been given to
experience the regenerating influence of Nature upon the
human intellect, will acquit me of all suspicion of cant, when
I say that above and beyond any honours or distinctions
which it is in the power of any academical body to bestow,
nay even beyond and above the appreciation of one's work
by one's own contemporaries, research confers its own reward
in the healthy habit of mind which it induces. It, and it
alone, can teach us to appreciate the sublime beauty of that
saying of Lessing's in his ' Streitschriften' with which the
President of the British Association closed his Address at
Manchester in 1887.

To get a glimpse of new fragments of truth before they
have become the fashionable idol of the academic crowd, or
the current coin of the examination-room, or the common-
place commodities of the publishing mart, is perhaps the
purest pleasure of which on the intellectual side our present
organization is capable.

A. I.

WELLINGTON COLLEGE,
BEEKS.

1st June, 1889.

CONTENTS.

i -INTRODUCTION. General and preliminary remarks

the term ' Metamorphism ' ... ... ... ... ... page 8

Divisions of the Subject .. . ... ... ... 4

ii PARAMORPHISM OR MINERAL CHANGE.

a. Primary Par amorphism (Genesis of Rocks} ... ... ,, 6

The necessity of a quondam universal glowing magma

at an early stage of the Earth's evolution ... ... ,,22

p. Secondary Paramorphism including a review of facts
and observations contributed by Judd (f), Allport
(g), Becker (i), and others Action of saline
(marine) waters in producing secondary minerals
the case of Grinshill fault (k) ,,24

iii METATROPY. The term defined and illustrated
Influence on crystalline form of

(1) Temperature 34

(2) Molecular water ... ,,34

(3) An accessory mineral ... ... ... ... 35

Vitrification and devitrification (instances considered) .. ,,36

Sulphur and phosphorus... ... .. ... ... 36

Arsenious oxide ... .. ... ... .. ... 38

Silica ... 39

Borax ... ... . . ... ... .. .. 40

Principles inferred .. ... ... ... ... ... 41

Behaviour of artificial glasses ... ... ... . ,,42

Influence of pressure on crystallization .. ... ... ,,46

Passage of minerals through the solid-liquid 'critical state ' 48

Action of water along junction-planes ... ... ... 52

iv METATAXIS.

Cleavage 55

ft. Crumpling ... ... ... ... .. ... ... 61

y. Foliation 62

ft Metataxic work done by Solar and Lunar Tides ... ,,65

Origin of some ' Augengneisses ' ... .. ... ,,67

Parallel case of ' glacier-ice ' considered ... ... ,,69

v HYPERPHORIC CHANGE 71

vi CONTACT-METAMORPHISM ,,74

First stage. Direct effects of heat and pressure ... ... ,,74

Second stage. Effects of the circulation of super-heated

water ... ... ... ... ... .. .. ,,77

Observations of Rosenbusch ... ... . .. ... ,,81

Third stage. Changes following upon cooling ... ... ,,82

Cases considered, as described by

Teall 84

Heim 85

Lehmann ... ... ... ... .. ... 85

Allport 85

The alleged inetamorphic origin of Granite ,,86

CONTENTS. XI.

GENERAL REMARKS ON METAMORPHISM page 87

The cases of the Todi-Windgallen Group 89

the Val Orsina (Vernayaz) Series ... ... 90

the Huronian ... ... . ... ... 90

the Erzgebirge . . . ... ... ... ... ,, 91

Application of the term ' metamorphism ' to the prevalent

morphological characters of the Archaean Series? .. 92

CONCLUSIONS .. 94

Appendixi. Notes on Laboratory Work ... ... ... ... 98

a. Sulphur 98

Latent heat of vitreous sulphur ... ... ... 98

b. Phosphorus 99

c. Silica 100

d. Solvent action of the humus-acids ... .. .. 101

e. Devitrification of Flints ,,103

Appendix ii.

Note A. Eeduction and Dissociation in Volcanic Action 106

B. Vitality and crystal-building ... ... ... 107

C. Hypothesis of a metallic kernel ... ... ... 108

D. Wet and Dry Reactions 108

E. Hypothesis as to ' Waves of Heat ' ,,109

F. The terms 'vitreous ' and 'amorphous ' ... ... 109

G. On Fritting 110

H. Orographic Structure of the Alps .. ... 110

I. On Serpentinization, &c. ... .. ... ... 114

K. Pfaff's doctrine ,,115

,, L. Genesis of Diamond and Graphite ... ... 115

M. Fossil evidence of extension in direction of

cleavage-dip .. .. ... ... ... 119

,, N. Von Cotta's observations at Predazzo ... ... 119

O. The Moon's Surface ,,120

P. The case of the N.W. Highlands ,,122

Q. OnSpinells ,,126

R. Fahlbander 127

S. Relation of Organic Developement to Physical

Environment ,. ... ... ... ... 128

T. The Airolo-series of Dr. Grubenmann ,,129

U. Prof . Credner's summary of metamorphic theories

(translated from his ' Elemente der Geologic,'

6th edition, 1887) ,,133

" Nur die iiberzeugendsten Griinde, also sowohl der
Nachweis der Ursache des Umwandlungsvorganges, als die
Beobachtung inniger, durch allmahlige Uebergange erwies-
ener Verkniipfung des umgewandelten Gesteines mit dem
urspriinglichen Muttergestein geben die Berechtigung, ein
Gestein als metarnorphisch zu bezeichnen."

PEOF. HEKMANN CEEDNER;

Elemente der Geologic

ROCK-METAMORPHISM

CONSIDERED FROM THE

CHEMICAL AND PHYSICAL SIDE.

1. INTRODUCTION.

IN Physical Geology there is no subject more complicated than
Metamorphism, nor one which presents to the student of
Geology so many vexed and complicated questions. True
progress in this direction can only be made by a threefold
attack upon the problems which it presents to us. They
must be studied

1. in the field ;

2. with the microscope ;

3. in the light of known physical laws and chemical

principles.

In the first of these three lines of work our own country has
undoubtedly taken the lead, but it has been rapidly overtaken
by the geologists of Germany, Austria, France, and America.

In the application of the microscope and microscopic methods
to petrology it would be admitted, I think, on all sides that we
have to thank Germany* mainly for the elaboration of this
more exact mode of research ; but thanks to the indefatigable
energy of several workers e.g., Bonney, Sorby, Allport, Judd
such rapid strides have been made in this country in the
last decade or so that English petrology may be said now to
take a place in this respect second to that of no country in the
world.

With respect to the chemico-physical side however it would
be rash to assert as much. There is some advance here in the
latest text books ; the physical and chemical sides of many
geological phenomena are handled much more freely than they
used to be ; but there is room yet for much improvement. It
is not the " text-book taster " in chemistry or physics who
will rise to that masterly handling of such problems which we
see in some of the best geological works on the Continent,
but the investigator who can bring such living ideas to the
consideration of them as can only be acquired through bond
fide laboratory-work.

It is not too much however to hope that, with the new
impetus which has been given of late years to laboratory-

* Zirkel's earlier work on the * Basalt Rocks ' (Bonn, 1870), is dedicated to
Henry Clifton Sorby, F.R.S., who initiated microscopic petrographical methods.
B

2 EOCK-METAMORPHISM.

work at the universities and the increased facilities which
are now afforded by our provincial colleges, the next few
years will be marked by rapid strides in this direction ; and
that the day will soon pass away in which we shall find
Bischof quoted as a final authority on such questions ; or hear
a professor of geology excuse himself for not having made
himself acquainted with memoirs on interesting and important
questions in physical geology on the ground that they are
' too chemical '; or find a distinguished author of a text-book
stating, after writing about dolomites, that " we are still quite
in the dark as to the exact nature of the reactions by which
they have been produced "; or find another eminent, but now
deceased, professor invoking the notion of the " spheroidal
state " of water to account for paroxysmal explosions at the
mouth of a volcano in activity ; or the reducing action of heated
protoxides of the heavy metals upon steam overlooked as it
generally is when the suggestion of Davy,* as to one possible
mode of production of free hydrogen at the mouth of a
volcano, is considered ; or the discussion at the Geological
Society of the chemical evidence which may bear upon a
stratigraphical question a practical impossibility ; or a learned
President of that same Society attempting to extend our
ideas of those differentiated forms of energy which are con-
cerned in the vital activity of living organisms to the build-
ing up of a crystalline mineral. t

General and Preliminary.

The principle of Conservation of Energy is a recognition of
the fact, that so far as the material universe is concerned
the sum-total of its energy is a fixed quantity. This energy
when manifested in operation is differentiated in the various
ways or modes to which we apply the term force, so that the
forces of nature may be defined as differentiated energy.

Without entering into the psychical side of the question we
may regard vitality in a living organism, so far as its physical
side is concerned, as a summation of forces.

When a selective differentiation of the forces essential to
vitality takes place in connection with special organs (i.e.,
differentiated structures), powers or capacities are developed ;
and a summation of powers constitutes that indefinable thing
which we call the individual.

To give a full account of the individual in any grade of
existence we must trace back each intermediate power which
is an essential constituent of the individual to those simple
elementary principles which we can recognise as laws uni-

* See Appendix ii. Note A. t Appendix ii. Note B.

INTRODUCTION. 3

versal. A scientific imagination may be useful in projecting
an idea ; but to call in that faculty in support of an idea, when
direct evidence from nature fails us, is whatever it may be
certainly not scientific. No true advance can be made by
such a use of it. No piling-up of opinion upon opinion can
establish a truth of nature, unless such opinions rest ultimately
upon evidence furnished by Nature herself.

In the conflict of opinions and views which has been
waged for years over those phenomena connected with rock-
structure which are generally understood to be included under
the term " metamorphism " in its more restricted sense, it is
to be feared that many " theories " which have been put
forward from time to time are simply suspended in the air ;
instead of hanging on to a series of rigid inductions from facts
they have often little more than the imaginations of their
authors to rest upon.* A really solid basis for theory can
only be laid in a careful and laborious observation by
impartial minds of the facts presented to us in nature, or
arrived at through the experimental work of the laboratory.
"We must get our ideas our way of looking at things our
"theories" by inductions from the hard facts of nature;
and this some of our best workers are beginning to realise.
But we must not imagine that an inference drawn from a set
of facts of one kind is of itself sufficient to give a full account
of the manifold and complex phenomena presented to us in
the more highly (so-called) metamorphic rocks, such as the
crystalline schists.

The term " Metamorphism."

The root idea of /AO/><^ is no doubt shape or form. But in
scientific nomenclature the idea is limited to internal structure.
In Botany, for example, there is no difficulty in distinguishing
between morphology and external conformation. The former
the structure and growth of the cell, the mode of elaboration
of tissues and organs is determined by definite laws and the
operation of definite forces (not in every case clearly defined, it
may be) ; that is to say, differentiated forms of energy applied in
Nature's laboratory. The gardener trains a fruit-tree to a wall,
and thus alters its external conformation ; but no one would be
so foolish as to say that the morphology of the plant was
changed thereby. It is equally unscientific to attempt to
extend the word metamorphism in petrology to such accidental
changes in the conformation of a rock-mass (large or small),
as may occur, for example, in the indentation of pebbles by

* See Appendix ii. Note E.
B 2

4 ROCK-METAMOEPHISM.

the pressure of a harder pebble upon a softer one, as is
often seen to be the case in the Nagelfluh conglomerate and
elsewhere.

How necessary it is to fix such limitations to the meaning
of the term is seen from the fact that a learned professor in
this country * only five years ago brought forward a similar
case from the Old Bed Sandstone Conglomerates of Scotland,
as furnishing what he called " an example of an early stage
of metamorphism."

In a letter dated Zurich, 1849, Von Cotta tells us that
Escher v. d. Linth showed him a great case full of such
in the Museum at Zurich. Many of the larger indented cal-
careous pebbles were split and their cracks filled with calc-
spar, just as the cracks in the septaria of the London Clay
are filled up. No one has yet ventured to bring forward the
latter as instances of ' metamorphism,' nor on the other
hand can it be shown, I think, that any continental writer has
indulged in such an abuse of the term, although some of them
at least as is shown above have long been familiar with the
phenomena. As well might we call the striations and grooves
of glaciated rock, or the work of a mason's chisel upon a
block of stone, ' metamorphism,' as to apply that term here.

In this thesis ' metamorphism ' will be used to mean only
changes in the internal structure o/ rock-masses (i.e. in their
morphology); everything connected with external conformation,
which is purely accidental, is excluded.

In an attempt to deal with the vast subject of meta-
morphism from the chemical and physical side, as thus out-
lined, it is not possible within the limits of a thesis to do more
than touch upon its more salient points. One thing however
must be premised ; we are not dealing merely with the so-
called ' rnetamorphic rocks ' of the systematist, but rather
with principles. For this reason it will be best to rise above
the text-book level of looking at the facts and ignore the
restrictions and limitations which may be convenient and even
necessary in classifying rocks. Here as in so many cases we
have to recognise the fact that Nature knows no sharp lines of
demarcation, though for the conveniences of study and de-
scription they are admissible.

Divisions of the Subject.

A year or two ago (see Brit. Assoc. Report, Birmingham, 1886,
p. 658) I proposed to exclude from metamorphism in the
stricter sense of the word, as defined in this work, all such
changes as could not be included under the two terms

* Brit. Assc. Report, Southampton Meeting (1882), p. 536,

INTRODUCTION. 5

" Metatropy " and " Paramorphism." Further consideration
of the subject has led me to see the necessity of recognising
in slaty cleavage and its concomitant phenomena a kind of
metamorphisrn ; and this will be considered under the term
Metataxis.*

We have therefore to deal with the three following :

1. Paramorphism, including all those changes within a
rock-mass (essentially of the nature of chemical change) in
which the original minerals have had their chemical composition
more or less altered, while new minerals are formed within
the mass.

2. Metatropy, or changes in the physical characters of
rock-masses, while there is no essential chemical change either
in the rock-mass or in its constituents.

3. Metataxis, or changes of order of the constituents of the
rock-mass, of which the phenomenon of slaty cleavage may be
taken as a typical instance.

As changes of the first class are atomic (chemical), and
those of the second-class molecular (physical) ; so changes of a
metataxic nature must be considered purely mechanical.

We may parallel the three kinds of change with the dis-
tinction which is generally and easily recognised in science in
the three degrees of divisibility of matter between

(a) the dissociation of a molecule into its several atoms ;

(b) the breaking up of a solid into a liquid and ultimately
into a gas, by elevation of temperature, the cohesion
of the mass being partly overcome when it assumes
the condition of liquidity, and entirely overcome when
it assumes the gaseous state ; i.e., when the mass is
divided into single and individual molecules ;

The distinction is as clear and as easily drawn in the one
case as in the other. Mere alteration of the external con-
formation of a rock-mass (e.g., flexure) has been already
excluded from the term metamorphism, and consequently
from each particular division of the subject.

There is yet a fourth class of changes which have often been
referred to by many writers as " metamorphic," but which are
scarcely admissible within the limits of the term as here laid
down. The fact that they have been thus described neces-
sitates some notice of them in the present work ; and for
these I propose the term " hyperphoric change." Such changes
as the introduction of a new mineral into, or the removal (wholly

* This term is preferred to the cognate term Metastasis (Bonney), since
that term has already been appropriated in Morphological Botany.

OF THB

UNIVERSITY

6 ROCK-METAMORPHISM.

or in part) of an old mineral from, the original rock-mass (of
which the dolomitization of limestones may be taken as typical)
fall under this head.

How necessary it is to have a nomenclature for discriminating the various
phases and degrees of "Metamorphism" is seen by the frequent confusion of
the argument arising from the want of such discrimination in more than one
of our recent text-books of physical Geology. Such terms as 'normal' and
'regional' metamorphism do not help us much, since they rather imply
that the cause we are in search of is known ; and they admit of a certain
element of vagueness arising from differences in the theoretical views of
those who use them. What is wanted is a terminology based on distinction
of kind rather than on genetic theories, and this must be offered as an excuse
for the new nomenclature here proposed.

il. PARAMORPHISM OR MINERAL CHANGE.

a. Primary Paramorphism (genesis of rocks). It may be well
to consider here in limine a few of the simpler chemical
reactions which have a direct bearing upon this part of our
subject, such as the decomposition of an original silicate, from
which silica may be deposited. In the formation of dolomite
again paramorphic change comes into play to some extent, and
there is no valid objection that I can see on chemical grounds
to the theory that both the carbonates of lime and magnesia
may have been precipitated in some cases in concentrated marine
waters, or waters of salt lakes, by chemical reactions.

To account for the enormous quantity of NaCl in sea-water
by mere washing-out of this mineral from the earth's crust
would be to reason in a vicious circle : it is more likely that it
has resulted partly from the action of more strongly electro-
positive elements of the alkalies (in this case mainly sodium)
taking up the strongly electro-negative chlorine from the
soluble chlorides of the alkaline earths and other bases. Thus
sodium or potassium dissolved out of felspar by carbonated
waters would be carried down to the sea in the form of soluble
carbonates. These would then re-act upon any chlorides of the
alkaline earths present thus

Na 2 CO 3 + CaCl 2 = 2 NaCl + CaC0 3 ;
K 2 C0 3 + CaCl 2 = 2 KC1 + CaCO..

In such cases there might be a direct precipitation of CaC0 3
to form limestone, and such reactions in the earliest condensed
waters may have been the origin of the bands of marble which are
met with in the crystalline schists of the Alps and elsewhere.

The decomposition of silicates by corbonated water is clearly
not the only source of the alkaline carbonates. If we con-
sider the pyrogenic stage of the formation of minerals in an
early stage of the earth's lithosphere, and take into account
the high degree of stability which the carbonates of the alkalies

ROCK-GENESIS. 7

manifest (infra) even under our present atmospheric pressure,
there seem strong grounds for assuming that under a much
greater pressure and at higher temperatures the alkaline car-
bonates would be freely formed in the dry way by direct
synthetic combination of CO 2 with the oxides of the alkali
metals, which, having a far higher degree of stability than
the carbonates, would have been already formed at a still
earlier stage.* The avidity with which halogens (in different
degrees) attack metals, the great thermal stability of their
chlorides for the most part and to some extent also of their
bromides, the differentiating factor of the non-solubility of
many of the fluorides, together justify us in assuming the
antecedent formation of a much greater variety and abundance
of haloid salts (especially of the chlorides) in the earliest
primaeval waters of the globe. t

Such considerations lead us to regard as concomitant and
correlated phenomena]: the occurrence of bands of crystalline
limestone among the true crystalline schists of the Alps (of
which I speak from personal knowledge) and elsewhere and
the initiation of that salinity of oceanic waters with which
we are familiar, long antecedent to the earliest reprecipitation
of Na Cl, KC1, &c., from them at later stages by local concen-
tration. And further they offer, I venture to think, a more
sufficient explanation than any I have yet met with of the
occurrence first noticed by Sorby of the presence of " chlorides
of potassium and sodium, and even of sulphates of potash,
soda and lime " in solution in the water of the 'fluid cavities'
of the quartz of granite ; traces of these salts being present in
the water included in the quartz during the original cooling of
the granite. In this sense we may perhaps understand that
distinguished observer's conclusion that it was "a genuine
constituent of the rock when melted."

The precipitation of Ca COs to form bands of marble in the schists, is not
to be understood as implying condensation of water on the crust on any large
scale. Such water as then existed must have formed mere puddles as
compared with the oceans of later time. Ca COs and Mg C0 3 were no doubt

* It will be recollected that the highest artificial temperatures obtainable are
incapable of dissociating the oxides of the alkalies and alkaline earths.

t See further Credner, Elem. der GeoL (6th ed). p. 321.
'The non-solubility of many of the fluorides.'
In 100 parts of water

Ca F 2 is dissolved to the extent of '037 parts.
Sr Fa rather more.
Ba Fa rather more than Sr F 2 .

The solubility increases here (as is so often the case) with the chemical
activity of the base. Take again the fluorides of the magnesium group of
metals (MgF 2 , ZnF 2 , CdF 2 , BeF 2 ) : of these only BeF 2 is at all freely
soluble.

t That is to say, co-ordinate products of the same chemical reactions,

8 ROCK-METAMORPHISM.

produced in early archsean times both by wet and dry reactions, though both
of course at high temperatures. The case of the calcareous schists of the
Glockner Group (in which the calcite is interfoliated with the quartz and the
inica) may perhaps serve as an instance of the formation of Ca CO 3 in the
wet way, and possibly by the action of free CO-2 in solution, since the large
quantity of free quartz present seems to point to this.

An analysis in my laboratory of the specimen of marble from the schists of
the Alps (the Valser Rhein, see App. ii. Note H.) gave 27% of siliceous and
earthy material insoluble in HC1. The specimen itself shows traces of a sort
of bedding-lamination both macro- and microscopically. Under the micr. a
thin slice exhibits opaque (earthy) material with a more or less linear arrange-
ment in a clear field, the latter transparent in ordinary light, but breaking up
into the finer crystalline-granular texture of 'korniger Kalk' between crossed
nicols. Its composition and texture thus combine to point to its probably
resulting from the reaction of an alkaline carbonate upon a silicate containing
lime and alumina as bases, the CaO taking up its equivalent of the COg while
the AtaOs was separated out, from its inability to take up COa under the
existing conditions. The marble occurs in lenticular masses.

In the earliest stages of the earth's history the chlorides may have been
formed by direct combination of Cl2 with the metals; but, looking at the
rapidity with which Cl2 replaces in Hg O at red heat, we may regard it as
far more probable that most of the Cl2 (and almost certainly the F 2 ) combined
directly with free 2 ; since this could take place at a higher temperature than
H0 + C1 2 = 2HC1 + O. As condensation advanced H F and H Cl dissolved in
H2 O would attack the oxides, silicates and carbonates of the metals and
furnish another abundant source of the chlorides in the earliest marine waters.

Prestwich's idea of the slow transformation [H20 + Cl2 = 2 H Cl + O] of
chlorine-water under the influence of light is true as far as the chemical fact
goes, but scarcely admissible under the high-temperature conditions we are
considering.

Probably both Br2 and 12 (which can be made to combine directly with H2
by heat and contact-action) united in the first instance also directly with H2 ;
their haloid salts in sea-water resulting from reactions of the acids HBr, HI
upon oxides, sulphides, carbonates, and silicates of the alkalies, alkaline earths,
and magnesium.

But we need not stop here. Carbonate of lime formed as
above suggested, containing, as it does, a more strongly electro-
positive base (calcium) than magnesium, its metallic base
would replace that metal, forming haloid salts of Ca, in the
place of corresponding haloids of magnesium, in the earliest
waters, thus

Ca C0 3 + Mg C1 2 = Ca C1 2 + Mg CO 8 ;

Ca CO 8 + Mg Br 2 = Ca Br 2 + Mg C0 3 .
(Mg Br 2 it will be recollected is the chief salt in " bittern "
from which bromine is obtained) . The haloid of calcium would
be removed in solution and the Mg C0 3 precipitated. These
things happening together and magnesium being remarkable
for its readiness to form double salts (which is a chemical fact),
there ought to be no difficulty in seeing how at a later stage
the double carbonate (dolomite) may in some cases have been
produced as the direct result of simultaneous chemical re-
actions, and even as a result of the reactions of the salts of
sea- water upon a pure deposit of carbonate of lime. Magnesium

EOCK-GENESIS. 9

sulphate in sea-water may react equally well on carbonate of
lime, thus

MgS0 4 + CaC0 3 = MgCO s + CaS0 4 *

This may explain how it is that gypsum is so frequent an
accompaniment of dolomite as it is in the Zechstein of
Germany. A study of the Durham coast-section in the summer
of 1886 forced upon my mind the idea, that much of what is
observable there in the rock-structure on the face of the older
parts of the cliff, so different from what is seen in new expo-
sures of the same beds in quarries very near the coast, is the
result of changes partly hyperphoric partly paramorphic, pro-
duced by the long continued action of the spray of the sea. It
throws some light too, I venture to think, upon the occurrence of
dolomites as well as marbles among the Alpine schists. Of course
it will be readily understood that these and analogous reactions
may be produced by salts held in solution, in mineral waters.
In this way local deposits of dolomite may have been produced.
Later experiments of Dr. Sterry Hunt have removed the
difficulty which was raised by the experimental failure of
Bischof in his attempts to precipitate pure dolomite. The
former investigator finds that they unite together to form
dolomite at the moment of their formation, though magnesite
could not be taken up in this way by bi-carbonate of lime in
solution. The formation of pure dolomite as a chemical pre-
cipitate is thus seen to be an illustration of the principle which
has gained recognition in modern chemistry, that radicles as
well as atoms have a special degree of activity in the ' nascent
state ;' since at that moment the system of which a radicle
consists has had its most stable form of configuration disturbed,
and so allows more independent action of its constituent atoms.

It is more probable that MgCOs was in such cases formed simultaneously
with CaCO 3 by direct action between alkaline carbonates and haloids of Mg
as well as Ca; for this simply assumes the presence of (e.g.] CaCla and
MgCb together in the same concentrated waters. This would give us the
antecedents necessary for Sterry Hunt's reaction mentioned above (p. 9).
The more intimate admixture of MgCl2 and CaCla in a concentrated solution
would render this process far more likely than that MgCa(COs) 2 should have
been produced by the direct action of COa upon the insoluble silicates of Mg
and Ca. (infra, pp. 12-13). There is also some difficulty in postulating the
action of MgSO-4 upon CaCOs in the early archsean times, on account of the
(thermal) instability of most of the sulphates, though this manifestly raises no
difficulty in a modern case like that of the Durham cliffs, nor does it cause
any difficulty in attributing archsean dolomitization to reactions of haloids of
Mg upon CaCOs as shown in the equations given on page 8 of this work
It is not improbable that the removal of a large amount of Mg from the
waters of the ocean during the vast interval between the Old Red Sandstone
and the Jurassic periods and the storage of it in the dolomites, by the
reactions mentioned on pages 8 and 9, may have done much to render the sea

* cf. Sorby, Q.J.GLS., vol. xxxv. p. 73. Such experiments would be more
satisfactory if checked by analyses of the solutions afterwards.

10 BOCK-METAMOBPHISM.

fit to support the teeming life of the Mesozoic ocean. I have verified these
by laboratory- work, and found

(1) That when the two mixtures were allowed to stand for two or three

weeks in distilled water, with occasional stirring, very small
quantities of Ca were found in the alkaline filtrate : but

(2) That on boiling for a couple of hours (Ca COs in excess), Ca was

copiously precipitated from the alkaline filtrate. [A 'blind
experiment' was then made with complete negative results, in
order to guarantee the absence of Ca in the filtrate from any
decomposition of Ca COs by mere boiling].

The condensation of the water of the Dead Sea (Sp. Gr. at a depth of 300
metres = 1*25, yielding 27 '8 % of solid residue) in which carbonates are absent,
and the salts in solution are almost wholly composed of haloids of K, Na, Ca,
Mg, is a remarkable fact. Had the physiography of the Jordan basin been
such as to charge its water more freely with alkaline carbonates the large
proportion (Roth, Attgem. u. Ch. GeoL, p. 478) of haloids of Ca and Mg now
suspended in its water would doubtless have been precipitated as carbonates to
form dolomite.

That class of paramorphic mineral changes by chemical
reactions, of which the formation of dolomite may be con-
sidered one of the simplest examples, would include, as Credner
remarks (EL der Geol, 3rd ed. p. 295), the ultimate conversion
of rocks containing augite, mica, hornblende, garnet, diallage,
olivine and chondrodite, and especially of eklogite, olivine-
rock, diabase, gabbro, diorite, hornblende-schist into ser-
pentine. This happens, he goes on to say, not merely as a
residuum from the processes of decomposition and removal in
solution by the action of carbonated waters, but chiefly through
the separation-out from solutions of carbonate sulphate and
chloride of magnesium, as soon as these come into contact
with silicates of the alkalies, alkaline earths, and alumina.
In all these the chief agency would seem to be heated or super-
heated water acting through a long period of time :

1. as a direct solvent for the extraction of certain salts from
the original minerals ;

2. by furnishing, as the result of the first, saline solutions
capable of acting upon other minerals either by direct synthesis
or by double decomposition. See ii. (/3).

Thermal Chemistry (as exemplified below in the carbonates
of the alkalies) would seem to throw some light upon the order
of succession of primary paramorphic changes from several
considerations, of which some may be here suggested :

1. The law that most heat is set free in the formation of the
most stable compounds. In so far as this heat escapes there is
a dissipation of energy, the heat lost representing an equivalent
abstraction from the total of the potential energy of chemical
affinity of the terrestrial mass. There must have been therefore
a gradual transition from the less stable to the more stable forms,
and when these were ultimately reached no further paramorphic
change could occur without alteration of physical conditions.

BOCK-GENESIS. 11

2. The great stability of the silicates points to a high degree
of probability that in the secular cooling of the earth's
original non-differentiated mass they were among the earliest
compounds solidified, extensive consolidation of the metals *
having preceded even this stage ; and generally we may say
that the relative stabilities of individual minerals of the earth's
crust must have been the prime factor in determining a general
progressive order in the deposition of them. The carbonates,
on account of their comparatively low temperatures of disso-
ciation, must have been among the later.

3. The high temperature (that of white-hot platinum) at
which steam is only partly dissociated f under the pressure of
our present atmosphere shows that the oxidation of hydrogen
must have taken place at a very early stage to form steam
immensely superheated under the pressure of an atmosphere
many times greater than that of our present gaseous envelope,
and at a period long prior to the first liquefaction of that body
on the globe.

Here then were the conditions, heat, gaseous water, pressure,
favourable to paramorphic changes in the first-formed
silicates, and giving, it would appear, strong support to the
view lately propounded by Prof. Bonney as a result of field
observation and microscopic work, " that the crystalline
schists (as he has limited the meaning of the term) and
gneisses were formed in archsean ages under conditions which
have never subsequently occurred in any large area of the
globe." Again as secular cooling went on, and the C0 2 began
to be fixed, carbonates of the alkalies would be formed, later on
those of the alkaline earths, carbonates of the heavy metals
being formed at a much later stage. This is a necessary de-
duction from the known relative stabilities of the carbonates ;
those of sodium and potassium melting at a red heat, and that
of potassium subliming in stronger heat, and at the same time
undergoing dissociation only to a slight extent ; while the
carbonates of the alkaline earths (of which that of lime, the
least stable, undergoes dissociation at the temperature of the
kiln under the pressure of one atmosphere) are intermediate
in stability between those of the alkalies and those of the
heavy metals. Now we know from laboratory experience that
carbonates of the alkalies decompose silicates readily by dry
fusion (converting these in some cases into the corresponding
carbonates) ; and it is probable that as a result of such re-
actions the first carbonate of lime made its appearance upon
the globe.

* See Appendix ii. Note 0.
t See Appendix ii. Note A.
Pres. Address, 1886.

12 BOOK-METAMOEPHISM.

The formation of carbonates of the alkaline earths when their silicates are
fused with alkaline carbonates we should expect to occur from the fact that at
a dull red heat carbonate of lime is formed by direct synthesis [CaO + COa =
CaCO 3 ]. This can easily be proved on heating a pellet of lime in an
atmosphere of C02 over mercury, by the loss of volume of the CO2 and its
liberation subsequently by stronger acids.

Wislicenus (Anorg. Chem. 378J says, "Kohlensaure Alkalien bilden mit
ihnen (silicates) beim Schmelzen unter Entwickelung von Kohlensaiiregas
und Abscheidung von Oxyden oder Carbonaten der Metalle in Wasser
losliche Alkalisilikate ;" and further that this is "nur vollkommen durch-
fiihrbar, wenn die betreffenden Silikate auf das feinste pulverisirt sind."

I have lately examined this point a little more minutely in my laboratory,
with the following results :

(1) Fusing together (until evolution of COa ceased)

(a) MgSi O 3 + Na 2 CO 3 + K 2 C0 3 ;

(b) Ca Si O 3 + Na 2 Co 3 + K 2 CO 3 ;

dissolving and completely washing away the alkaline silicates and the excess
of the carbonates until nothing but distilled water went through the filter;
the residue thus completely washed gives off C0 2 with considerable
effervescence when thrown into water and treated with a few drops of HC1.

(2) As an instance of what occurs with silicates of the less active bases of
the heavier metals, native silicate of copper (Kieselmalachit) was subjected to
the same treatment, the result being that the whole of the copper separated
out as oxide, as could be seen by the total absence of the green colour of the
carbonate in the washed residue, and its failure to give off CO 2 on treatment
with HC1.

On general grounds it is conceivable that under a much greater pressure
(say 100 atmospheres, which is not an extravagant range to take for the
atmospheric pressure of early archsean time), preventing the escape of the
COa as it is replaced by the SiO 2 , even carbonates of the heavy metals might
be formed; and with a much more moderate increase of the present
atmospheric pressures we should expect the proportion of CaCOa and MgCO 3
by dry fusion to be largely increased.

Wet and Dry Reactions. The important difference between 'dry and wet
reactions' is brought out so clearly by our experience of the behaviour of the
silicates, that this furnishes a good opportunity for emphasizing it.

(1) We know perfectly well that free CO 2 at ordinary temperatures and in
the presence of water replaces Si0 2 freely (as in the kaolinization of the
felspars) converting alkaline silicates into carbonates: on the other hand, at
fusion-temperatures (in the absence of moisture) this is reversed, and free
SiOa replaces CO 2 freely. This is seen to be an excellent instance of the
importance of 'physical conditions/ in determining the most stable form of a
mineral complex, as pointed out on p. 10.

(2) In the wet way, even with prolonged boiling (two or three hours), free
SiOa does not always replace COa in alkaline carbonates, though in some of
its allotropic modifications, as shown by my own studies (described further
on in this work) SiO 2 does so undoubtedly to a slight extent. But in double
decompositions at fusion-temperatures the SiO 2 passes over from a silicate of
a comparatively feeble base to an alkaline base, even to the replacement of
COa of the carbonate ; the COa escaping or uniting with the weaker base if
that is fairly strong, and perhaps with oxides of some heavy metals if confined
by pressure until the temperature of the mass is lowered below the
dissociation-temperature of their carbonates.

Now if we have regard to dry fusion, whether deep in the canal of a
volcano or as going on generally at and near the surface of the globe when its
outer sub-atmospheric lithosphere was for the most part in a glowing-liquid
condition (such traces of H 2 O as might be present being in the super-heated
condition of a dry gas and therefore not interfering with the conditions
requisite for dry fusion) we see at once the important differentiating function

ROCK-GENESIS. 13

of heated (molten) SiO2 in abstracting bases (especially CaO,MgO,FeO)
from such carbonates of those bases as might exist in (or be furnished to) the
mass before 'fluxing' ; and so a rational explanation is furnished of the
extremely silicious nature which both volcanic rocks and the earlier archaean
rocks (formed probably in the absence of wet water) possess in common.

On the other hand, after the cooling-down of such masses, and the access of
liquid water containing free CO2, this process (as noticed in ii. ft) was
reversed : CO2 replaced SiO2 in silicates of the alkalies, alkaline-earths, and
MgO ; carbonates of these bases were formed as substitution-products
(removed or deposited according as they were soluble or not in water) with the
separation-out of SiO 2 , at first hydrated, but ultimately passing into true
quartz with loss of H2 O.*

Of course the same temperature is not required in all cases for 'dry fusion' ;
this will be largely affected by the proportions of good 'fluxing' bases present.
Much information of great importance to the petrologist is given on this
subject in the volume On Fuel of Dr. Percy's 'Metallurgy' pp. 60 75.

A very good example of the reaction

CO 2 + Na 2 SiO 3 = Na 2 CO 3 + Si O 2

by the action of atmospheric CO 2 recently came under my notice, and I dare
say the thing has been often noticed before. Some solution of Na 2 Si 3 was
left at the bottom of a bottle, and the water gradually evaporated away in the
air of the laboratory. As this went on the walls of the bottle were corroded
by the separated Si0 2 acting upon the glass of the bottle, very likely to make
good some deficiency of the Si0 2 required for the complete chemical
saturation of the weaker bases (Al 2 Oa & FeO) in the bottle glass, (which
generally contains under 60% of SiO 2 ), and causing surface-devitrification of
the glass. The mass of the silicate when dry was found encrusted with a fine
growth of crystals, which by their free solubility in water and evolution of
C0 2 on treatment with a few drops of HC1 proved to be Na 2 C0 3 . (See iii.)

When such conditions were reached that steam could liquefy
and CO 2 could be held in solution in water, this water, becoming
highly charged with the gas under the then atmospheric pressure,
would attack the silicates of the alkalies, taking up those strongly
positive metals as soluble carbonates, with deposition of silica
(Si0 2 ) to form the qiiartz, which appears in such quantities in
the archoRan gneisses and schists. These carbonates of the
alkalies in solution, when brought into contact with haloid
salts of the alkaline earths and the heavy metals, would fur-
ther contribute to the formation and precipitation of their car-
bonates by such reactions as are given on page 6.

The earliest terrestrial waters are spoken of in the Thesis as being highly
charged with CO 2 'under the then atmospheric pressure.' This is not quite
accurately stated. Bunsen's investigations, with which most students of physics
are familiar, have established the law that where a liquid (in this case water)
is in contact with a mixture of gases, the weight of each gas (for a given
temperature) dissolved is proportionate to the pressure of that gas. Of the
existence of a much larger proportion of CO 2 in the archsean atmosphere there
can be no doubt. This point has been so strongly put by Credner, (Geologie
6th ed. p. 321) that I quote his own words: "In den altesten Perioden waren
die atmospharische Niederschlage kohlensaure-reicher als jetzt, da sie ihren
Weg durch einen Luftkreis machen mussten, in welchem sich die ganze

cf. J. A. Phillips 'On the History of Mineral Veins' (Q.J.G.S., vol. xxxv.)

14 KOCK-MET&MORPHISM.

Kohlensaure, die heute den Karbonatgesteinen als solche, in den Pflanzen und
Kohlenge&teinen, sowie im Graphit als Kohlenstoff, im Bitumen als Kohlen-
wasserstoff der Erde einverleibt ist, noch in gasformigem Zustande verteilt
befand. Diese an Kohlensaure reiche Regenwasser mogen auf die Kalk-und
Magnesiasilikate der Erstarrungskruste in hohem Grade zerlegend eingewirkt
und dem Meere stark konzentrirte Solutionen von Kalk- und Magnesia-
karbonaten zugefuhrt haben."

In Note L (App. ii) I have shown how our laboratory-experience leads us to
see the probability of the production of the archsean graphite by dissociation
of hydrocarbons ; but with this exception we may accept the idea contained in
this passage in its entirety. The same writer points out how the absence of
organisms capable of secreting carbonate of lime from these primeval waters
would be favourable to their local concentration ; he also notes that the water
of the present ocean requires evaporation to the extent of 75% before
precipitation of CaCO 3 sets in.

Accepting Bischof 's statement " that the carbonate of lime of all the forma-
tions would form a layer over the Earth 1000 feet thick," Pfaff (Geol. als
Ex. Wiss., p. 162} has calculated that the COa contained in it alone would in
the free state give a pressure of 356 atmospheres at the surface of the
Earth. This is probably excessive if in the calculation the force of gravity
has been assumed the same as we know it.

The action of CO2 appears to me to have reached its maximum in later
archsean and earlier palaeozoic times, the grauwacke of the former forming
perhaps a connecting physical link (with its authigenous mica) between the
archaean crystalline rocks and the true sedimentary palaeozoic series. This, I
find, has been recently pointed out by Kalkowsky (Elemente der Lithologie, p.
280.,) "Dass diese Gesteine (archaische Grauwacke, including 'Glimmer-
trapp') eine Briicke bilden, die uns zu den holokrystallinen archaischen
Gesteinen hiniiberflihrt, so dass iiber diese neues Licht verbreitet wird, ist
augenscheinlich."

I have adopted the terms 'authigenous' and 'allothigenous' from Kalkowsky
(Lithologie, p. 13^ because they seem exactly the words wanted. 'Endoge-
nous' and 'exogenous' are terms that appear to have found favour in some
quarters in this country ; but the two serious objections to their use in this
sense are, (1) that the connotation they bring with them from morphological
botany is misleading: (2) they have been appropriated in a different sense in
lithology (see note to vi).

May we not account for the occurrence of the authigenous micas and the
garnets in the cement of these grauwacke as resulting from the combination
of the nascent SiOz (replaced by free C02 as suggested in this work p. 13)
with the requisite bases sparingly present, while the large excess of the Si02
went to form the bulk of the grauwacke?

Such considerations point to a very unequal distribution of the silicates in
the earlier crust, as well as marked localization of the earliest condensed
waters; but the action of free CO2 may be regarded as perhaps the main
factor in determining the distribution of the earliest limestones and quartzites.
Where the CO2 attacked MgSiOa or CaSiOs (or both) impure (siliceous)
carbonates would be formed; but where Na 2 SiO 3 or K2Si0 3 were chiefly
present the carbonates formed would be removed in solution, and the SiO2
would furnish the mass of the material of a grauwacke.

The distribution of the limestones and quartzites (if such was the mode of
their genesis) shows the continued action of free C0 2 as a very potent
paramorphic agency in earlier palaeozoic times; while the theory dispenses
entirely with all necessity for postulating the existence of living organisms as
agents concerned in their production.

The fact that excess of C0 2 in solution in water causes CaCO 3 to enter into
the form of the soluble bicarbonate CaH 2 (C0 3 ) 2 may appear prima facie to
be a fatal objection to the theory that limestones per se could be formed in
any quantity by the direct action of carbonic acid on the lime- and magnesia-

ROCK-QENESIS. 15

silicates ; but the objection loses its force when we consider the further fact
that the bicarbonate is resolved into the normal carbonate, with separation-out
of CO2 and H2 O below the ordinary boiling-point of water (100C. ), and even
by concentration of the solution by slow evaporation at ordinary temperatures
as in the formation of stalactites. The physical conditions which cause
decomposition of the bicarbonate would prevent its formation.

From such considerations it is clear that we have no right to interpret the
crystalline texture of archsean and palaeozoic limestones ( ' marbles ') as of itself
furnishing proof even of extensive metatropic change ; since their crystalline
character may be diagenetic rather than ' metamorphic.' Even the crystalline
character of the limestones is thus seen to afford a very feeble support to
theories of ' regional metamorphism ' ; and it has also been shown that the
quartzites are not of necessity metamorphosed ' clean sandstones.' The
determination of these alternatives in any given case must clearly turn mainly
upon the microscopic examination of the rock ; it is enough for our present
purpose to show that an alternative explanation is possible.*
The order

Si Og, the non-metallic oxide of silicates.

COa ,, carbonates.

Halogens, the non-metallic elements of haloids.

S Oa, the non-metallic oxide of sulphates.

P2 65 phosphates.

gives the general relative electro -chemical energies of the chief acid
constituents which enter extensively into the formation of those mineral salts
which are widely distributed in the crust of the earth as rock-constituents.

It is not difficult to understand how individual atoms of the alkali metals
may have passed through a cycle of changes, aided by the ready solubility of
their salts in water and the facility which this fact furnishes for their
transport locally. Having first entered into combination with Si Og at a
glowing heat, to contribute to the formation of the earliest portions of the crust
by forming (thermally) stable silicates, on the solution of these silicates later
on in superheated terrestrial waters they may have taken up in succession the
other acids in the order of their electro-negative energies, at each stage of
change rejecting the feebler acid (to be taken up or not, as the case might be)
to enter into a more (chemically) stable state of combination, heat being
evolved at every step.

The importance of the part played in promoting paramorphic
mineral change by solutions of the alkaline silicates, whether
formed by the direct action of superheated water or by reaction
of carbonates of the alkalies upon the silicates of other metals,
may be seen from the following examples of reactions cited by
Credner (op. cit. p. 198 et seq.) :

(1) decomposition of the sulphates and chlorides of lime
and magnesia with the formation of their silicates ;

(2) decomposition of the NaCi by silicate of potash with
production of Sylvine (KC1) and silicate of soda ;

(3) decomposition of bicarbonate of magnesia with for-
mation of silicate of magnesia ;

(4) decomposition of ferrous carbonate with formation
of ferrous silicate ;

* While the above was passing through the press I read Prof. R. D. Irving's paper, "Is
there a Huronian Group ? " (Am. Jour. Sci., Nos. 201-203). The facts recounted by him in
his description of the Huronian series proper furnish apt illustrations of the view expressed
in the above paragraph.

16 BOCK- METAMORPHISM.

(5) decomposition of bicarbonate of lime by silicate of
soda, the semi-replacement of the sodium by the
calcium leading to the deposition of quartz as a
pseudomorph after calcite.*

The whole subject is full of interest to the student who can
bring chemical ideas to the study of petrology. It must be
borne in mind that all normal salts of the alkalies are soluble in
water, so that the new salts which those bases form are readily
transferred by water.

It will be seen that the suggestions here thrown out as to
the part played by superheated water and solutions of mineral
salts in promoting paramorphic change, taken in connection
with principles learnt from thermal chemistry, amount to
something quite different to the theory of ' hydrochemical
metamorphism,' over which more than one eminent geologist
of a generation now passing away has too blindly followed Bischof .
Hydrochemical action explains much, but not everything; and in
order to get a correct idea of its sphere of action, we must carry
our minds back to a remote past in the history of our planet, when
aqueous precipitation from the atmosphere was something very
different from that of the oxygenated and carbonated waters
of the atmosphere of later times upon which Bischof's theory
mainly depends. If the archaean gneisses and schists had been
formed by the latter agency at such enormous depths as the
extreme hydrochemical theory supposes, the time occupied in
their ' metamorphosis ' is represented by the whole of geological
time from the Silurian age down to the present ; since " all
the formations (as Credner remarks, op. cit. p. 307) from the
Silurian to the present are found, wherever their normal types
can be studied, without that extreme phase of ' metamor-
phism ' which characterises a true schist." And there are the
further insuperable objections found in the following facts :
(1) The fact that all the palaeozoic formations, and especially
those of the Cambrian System, contain rolled and worn
fragments of the archcean schists and gneisses in their con-
glomerates, which fragments had their several mineral cha-
racters at the time of their deposit in their present habitat ;
though they, together with the strata which now contain
them, and the primordial schists and gneisses from which
they were derived, have undergone further changes of a meta-
tropic and metataxic rather than of a paramorphic character
in the further metamorphism of so-called metamorphosed rocks.
We have to thank Bonney for elucidating this last point in one
of his recent contributions to science,! though the idea in a
crude form must have forced itself, I think, upon the minds of

t Presidential Address to the Geological Society, 1886.

BOCK-GENESIS:

"^^^^^y i L i r u n Vi^^^^"

other thoughtful observers of Alpine roc^piij^nTniTTena, as it
has upon my own. (2) There is the fact of unconformity which
is often very marked and of such frequent occurrence between
the archsean schists and the palaeozoic formations.

Such evidence goes a long way to show that the essential
features of the fundamental gneisses and schists features
which give a general character to the pre- Silurian groups of
strata often over 30,000 metres in thickness, wherever they
occur, in India, Scandinavia, in Canada and in the Alps are
such as are connected with the crystalline form in which they
were originally consolidated under conditions of temperature
and pressure very different from those which now prevail at the
surface of the globe ; and that the process of metamorphism
so-called, which distinctly characterises the pre- Silurian for-
mations, was at the entry of the earth into the Silurian Period
already completed, and could not therefore have occupied the
enormous period of time demanded for it by the advocates of
the extreme hydrochemical theory. Were it otherwise, all
our palasozoic formations must long ago have been converted
into crystalline schists and gneisses (Credner, op. cit. p. 307).
Comparing this with the conclusion at which Prof. Bonney
has independently arrived as published in his address of
February, 1886 we may congratulate ourselves as students
of physical geology that the more extreme theories of the
Huttonian school are breaking down, and that our foremost
British workers are beginning to join hands over this question
with those of the continent as represented by Giimbel, Cred-
ner, Pfaff,* and others.

The term ' Silurian ' which occurs several times on this page is to be under-
stood (in the sense in which it is now used by some of the continental writers)
as including the Cambrian.-^ This is not surprising when we recollect the
prominence of the author of the former term, and the difficulty which most
continental geologists must have felt in deciding upon the merits of the great
Murchison-Sedgwick Controversy, Even Ramsay seems to have found a
difficulty in sharply dividing the Cambrian from the Lower Silurian. (See
Phys. Geol of Gt. Britain, 5th ed., 1878, Table of formations, p. 30).

' The further metamorphism of so-called metamorphic rocks.' As illustrations
of what is here intended the following references are given :
In Prof. Bonney's Presidential Address (1886);
Fig. 1. Lepontine gneiss with subsequently-induced cleavage across the

original bedding.
Fig. 2. Calcareous mica-schist with similar cleavage.

* Pfaff's critical analysis of the arguments on which the advocates of ' regional metamor-
phism ' rely is well worth the attention of every student of physical geology, (op. cit. pp.
145 et seq.)

t Prof. R. D. Irviug fop. cit.) has shown plainly enough that the true Huronian has a
basal conglomeratic member as the Cambrian has. Making allowance however for pre-
Cambrian tidal action, I do not think we are bound to follow him in his deductions as to the
space in geologic time represented by the Huronian with its 18,000 feet of strata,

18 KOCK-METAMORPHISM.

In both cases filmy white mica is found on the cleavage-planes, and the
phenomenon is designated 'cleavage-foliation.'

Metataxis is recorded in the wavy contortions of the original foliation-planes
and the approximation of the flakes of mica to a parallelism with the cleavage-
planes.

Figs. 3 and 4 also furnish excellent examples of subsequent metataxic
alteration.

With these we may compare the statement of Allport (Q.J.G.S., vol. xxxii,
p. 426) that "there is evidence to show that the slates of the Cornish
Peninsula existed as metamorphic rocks (i.e. cleaved and in places contorted)
long before the intrusion of granite. There the contact metamorphism
extends to a short distance only (quite distinguishable by modern methods of
research from the previous metamorphism) the transition from the one to the
other being very gradual."

The importance of the unconformity referred to on p. 17 has been
emphasized by Prestwich ('Geology,' vol. i, p. 419).

( 1 ) The passage ' by insensible gradations ' of the ' early gneisses ' on the one
hand into granite and on the other into mica-schists, with which crystalline
limestones quartzites and iron-ores are instratified, is contrasted with

(2) the fact that " between the Archaean rocks and the succeeding Cambrian
series there is in Europe as well as in America a marked break of continuity."

How disappointing it is in connection with this subject to
turn to even the more recent of our text-books written by our
foremost geologists, may be seen by noting that in Geikie's
Text-book of Geology the archaean crystalline schists are
dismissed within a page and a half (pp. 588, 9). The foregoing
facts and arguments appear to lie altogether outside the mental
horizon of the distinguished author of that work. It is entirely
to miss the point to talk of these rocks having been formed
" during a period of the earth's history when the ocean had a
considerably different relative proportion of mineral substances
dissolved in its waters "; or, again, to speak of " the same
order (of chemical precipitation) having been followed every-
where over the floor of the ocean " ; and that for the simple
reason that the theory requires, and deduces from known facts
and principles, a prevalent set of physical conditions under
which the ' ocean ' could not possibly have existed as such *
The vast range of temperature between the first initial
oxidation of hydrogen f and the condensation of steam
into water on any general scale must have afforded ample
space in time in the cooling-down of the original nebulous
mass of the globe for the formation and deposition of
silicates and the separation-out of the quartz, which to-
gether make up the granitoid rocks, the gneisses, and the
crystalline schists; while on the other hand the high degree
of stability of most of the silicates removes all difficulty in the
way of their formation while, as yet, the temperature was too
high for the condensation of water in anything like oceanic

* cf. Pfaff, op. cit. pp. 25 et seq. t See Appendix ii. Note A.

ROCK-GENESIS. 19

proportions. These considerations lead us to contemplate
rather a condition of things in which deposition on a vast
scale was going on from the then dense atmosphere, so as to
form a growing non-consolidated lithosphere on the surface of
the globe an universal hot magma, which by gradually
crystallizing would leave traces of a sort of bedding arrange-
ment of its materials from differences of the specific gravity
of the minerals formed,* while the mechanical process of
stratification in aqueous basins (as the term is generally
applied) would be out of the question. Such a view, when
allowance is made for contemporaneous paramorphic change
by chemical reactions between the minerals, appears to be a
partial explanation of the banded structure of the archaean
schists, when viewed on a large scale, and the occurrence in
them of quartz-layers often of considerable thickness.

A similar incapacity for comprehending the theory here
advocated appears in Green's ' Physical Geology ' (pp. 426-27.)
Nothing is easier than to dismiss as ' dreamy conjecture ' what
we do not comprehend ; and Mr. Green's misconception of the
theory is shown by his attempt to state it in the following
words : " The constituents of those rocks were supposed to
have been held in solution in an ocean of boiling water, and to
have been precipitated as it cooled. " j-

I submit that the argument advanced in this section of the
present work proceeds by way of induction from known facts';
and that the further fact that there is a tendency in some
quarters to depreciate an argument which proceeds on
chemical and physical lines is no refutation of the theory
advanced.

Deformation of rocks by the shearing and crushing which
have inevitably accompanied the great earth-movements, whose
effect is traced in enormous foldings, fractures and overfolds,
(by the resolution locally of normal into tangential forces in
the contraction of the Earth's mass in the process of cooling)
has certainly brought about petrological changes ; but they do
not appear to have resulted anywhere in the formation of a true
schist or gneiss out of originally clastic rocks. This point is
more fully discussed later on. Gotta pointed out in some
remarks on the contorted strata about Vierwaldstadtersee (in a
letter dated from Zurich in 1849) that the strata which we now
see contorted must have been at the time in a more pasty con-
dition from saturation with water, and probably have acquired
their present rigidity by partial desiccation since their upheaval.
Yet even in this pasty condition pressure has not succeeded in
converting them into crystalline schists. No rocks on a large scale

* See Section v.
f The absurdity of this notion is exposed by Pfaff, (op. cit. p. 27.)

20 ROCK-METAMOKPHISM.

are equally saturated with water ; and in great earth -movements
it follows that with different degrees of plasticity (owing partly
to this fact and partly to differences of constitution) it would
necessarily result, that, while some portions were bent, con-
torted, and folded, other portions would suffer fracture, crushing,
and in places even pulverization in different degrees. In such
portions we should be prepared to find clastic materials present ;
but we have clearly no right whatever to infer from this fact
the derivation of a deformed crystalline rock-mass from
originally clastic materials. One of the main props of the
theory of extreme regional metamorphism is thus knocked away.*

The simulation of the macroscopic characters of true schists
by later rocks as the result, partly on the one hand of hydro-
thermal and chemical action, partly of deformation of rocks,
by pressure, crushing, and shearing on the other, leaves open
still a vast field for further microscopic research, united with
field work.t

Among the accessory minerals common in the oldest crystal-
line rocks the most widely distributed are apatite, rutile,
zirkoD. The occurrence of these very stable minerals as
accessories among the primal mineral constituents of the oldest
rocks (granite, syenite, gneiss, mica-schists) as well as in the
chief varieties of eruptive rocks, is entirely in accord with, and
thus far lends support to, the theory that relative stability has
been the main factor in determining (of course in the inverse
order) the general succession of mineral deposits in the forma-
tion of the earth's crust.

It is not contended that in this there has been anything
like absolute uniformity of succession. That we should con-
sider on a priori grounds extremely unlikely, because we have
no grounds whatever for assuming the thermal energy which
pervaded the non-differentiated mass to have been equally dis-
tributed throughout it.J And so it happens as theory
would lead us to expect that numerous irregularities are
observed in the general order of succession ; now this now
that accessory mineral being common in the various types
of the fundamental rocks ; schists occurring subordinately
among the gneisses, and gneiss occurring subordinately among
the schists; garnet taking the place of mica, and giving us

* Since this was written a remarkable instance of the effect of crushing
and subsequent decomposition of the pulverised materials in producing a
pseudo-breccia in the gneissic rocks of the Malvern Hills has been described
by Professor McKenny Hughes. (Geological Magazine, November, 1887).

t Kalkowsky (Lithologie, p. 43) remarks : " In Folge von Pressungen und
Spannungen bei der Bildung konnen Korper dieser Klasse (einfach brechende
Korper) doppelt licht-brechend sein, meist aber nur in schwachem Grade. Es
bedarf eines sehr guten Polarizationsapparates und eines unermiideten Auges,
um solche Spuren von Doppelbrechung wahrzunehmen."
^ See App. ii. Note R,

EOCK-GENESIS. 21

locally granulite in the place of granite ; gneiss containing in
different regions, some here others there, such accessories
as graphite, garnet, tourmaline, epidote, rutile, zirkon, horn-
blende, chlorite, apatite, micaceous iron-oxide, magnetic iron
oxide, pyrites ; the mica contained in gneiss though generally
inuscovite, being sometimes biotite (Erzgebirge) , as is the case
occasionally with granite (Dartmoor) ; hornblende in one
variety of gneiss, cordierite in another, augite in a third, garnet,
graphite/ 1 ' chlorite, green mica, all in their several turns,
replacing, or appearing side by side with, the mica, and giving
their respective varietal characters to the gneisses.

Yet through all this apparent confusion there is traceable a
leading principle in the oldest rocks general predominance of
the more acid minerals (the most stable) in the granitoid and
gneissic rocks, while, as we pass into the overlying schists and
phyllites of the archaean series, the more acid minerals gradually
give place more and more to the more basic minerals which
are somewhat inferior (as a class) in stability to the minerals
which preponderate in the granites and gneisses.

We have been here concerned (under the head of ' paramor-
phism ') with a number of mineral changes, of which chemistry
gives us some knowledge, among the materials which make up
the mass of a rock per se ; and we have not limited our view
to changes which might occur after the rock was once formed,
but have rather endeavoured in a firm belief in the continuity
of nature's principles of operation to trace back the working
of the principles learnt from physics, chemistry and mineralogy
to the primal genesis of the most ancient crystalline rocks,
the granitoid, gneissic, and schistoid rocks of the earlier crust
of the globe. In this we have arrived independently at some
general conclusions which are found to be to some degree in
accord with Gumbel's doctrine of ' diagenesis.' In all such
changes we may regard the mass of the rock as remaining
pretty constant in its composition after the primal accumula-
tions of its materials, notwithstanding the changes which may
take place reciprocally among its mineral constituents ; so that
a series of bulk-analyses would could they have been made at
the various stages of its history have given pretty constant
results. It is in this sense that one feels justified in the use
of the term " paramorphism " adopted here.

Primary paramorphism has thus been treated as having
to do mainly with the actual genesis of the rocks themselves
through the agency of mineral change ; and in this sense it
comes within the larger subject of " metamorphism." We
have been led to see strong reasons for regarding the processes

* See Appendix ii. Note L.

22 KOCK-METAMOBPHISM.

of primary paramorphism as having been determined by
conditions which must have prevailed universally and could
only have prevailed universally, at the surface of the globe at
a very early stage of its developeinent out of its primordial
nebulous mass.

The following laws and principles may be assumed as leading to this
conclusion :

1. The Law of Universal Attraction, (the force of attraction varying
directly as product of the masses, and inversely as the square of the
distance) ; and the specialized operation of this law in all cases of gravitation.

2. Elevation of temperature when latent heat is set free either in the
liquefaction of aeriform matter, or in the solidification of liquids.

3. Transformation of potential energy into kinetic energy, manifested as
heat, in chemical combinations.

4. Dissipation of Energy, a tendency to equalization of energy throughout
the universe when it is transformed into heat.

5. Transformation of energy of motion into heat in all cases of impact.

6. The retardation of radiation by such gases and vapours as are not
diathermanous, with which the researches of Tyndall have principally made
us familiar.

7. The enormous range of the condensation-temperatures of the several
elements known to us, from that of Osmium or Ruthenium (App. ii, Note C)
to that of Hydrogen gas, which our globe is far from having reached as yet.

These known and demonstrable laws and principles being assumed, it is not
difficult to see how very high-temperature conditions of the Earth's lithosphere
must have resulted at a certain stage of the evolution of the planet out of a
nebulous mass, producing in fact the 'Solar Stage' of the developement of
worlds. Recent researches in astronomical physics have so far verified the
Kant-Laplace hypothesis as to warrant the assumption that the early history
of the Earth has proceeded on such lines. Without attempting to follow Mr.
Crookes* in his speculations on the genesis of the elements themselves, it is
sufficient for our purpose to assume (as we may safely do) that, if this Earth
has developed from a nebulous mass, the matter of which it is made up must
have existed once in an extreme state of elemental dissociation.

Given then a nebulous mass of matter in such a state of elemental
dissociation and losing heat by radiation into space, a point must be reached
at which condensation of certain elements (those possessed of the highest
condensation-temperatures, and the least potential energy of chemical affinity)
must set in. As a direct result of this, concentration into a nucleus must
follow from the law of universal attraction, as certainly as we see two corks
run together when they float freely in water within such limits of distance
that their mutual attraction is able to overcome the feeble cohesion of water.
As the nucleus (the embryo-sphere) is thus formed, latent heat is set free,
and the temperature of the nucleus is raised, giving off its heat by radiation,
to be absorbed for the most part by the surrounding nebulous matter,
and ultimately lost by radiation into space. As dissipation of energy
progresses, further condensation must follow, the newly-condensed matter
gravitating towards the nucleus, every increment of mass in this increasing
proportionately the force of gravitation.

Equilibrium of pressure being upset in the mass by the resolution of the
general law of universal attraction into the specialized force of gravitation, a
certain rough stratification of the surrounding nebulous material must follow,
at first perhaps resulting mainly in the condensation on a large scale of the
vapours of the heavy metals, to form the outer zones of the growing

* See Address to the Chemical Section of the British Association, Birmingham Meeting,

1K86.

BOCK-GENESIS. 23

'barysphere' (Suess), while the tendency of pressure to facilitate chemical
combination (in other words to raise the dissociation-temperatures of oxides,
haloids, &c.), in the inner strata of the nebulous material, must lead to a
selective alteration of that material, tending in the direction of those
conditions which have given us the residual atmosphere of later stages of the
Earth's history. It would be easy to fall into the error of overestimating the
pressure near the surface of the barysphere at this stage, if we overlooked the
fact, that the mass of the barysphere must have been then less than
that of the present globe ; still there can be no doubt, from a consideration of
the general conditions of the problem that the hydrostatic pressure was many
times greater than that of our present atmosphere, though the value of the
"g" of our ordinary dynamic formulae was then probably less than 32ft. per
second. It may be predicated with a high degree of certainty, that as the
nucleus grew into the barysphere, and every increment of mass increased the
force of gravitation, condensation must have gone on loith great and accelerated
rapidity ; with such rapidity, in fact, that the liberation of latent heat from
the condensed matter alone would lead to a considerable elevation of
temparature ; and this would be immensely increased by the adiathermaneity
of a very large proportion of the aeriform materials of the surrounding
nebulous mass. Add to this combustion on a grand scale, as such elements as
hydrogen, the alkali-metals, the metals of the alkaline earths, aluminium,
magnesium, and at least the non-metallic elements, silicon, and phosphorus,
became oxidized in the inner nebulous strata, their oxides, &c., being
precipitated as further increments to the barysphere; and we may assert, as
positively as any inferential truth can be asserted, that with such an enormous
and rapid developement of heat at and near the surface of the barysphere and
in the rapidly-growing lithosphere, together with such retardation of radiation
into space as must have been caused by the nature of the dense nebulous
envelope which surrounded it, the developement of thermal energy qud heat
must have been for a considerable period of the Earth's history much more rapid
than its dissipation by radiation into space. Here then we have all the factors
needed for producing the high-temperature conditions required by the theory of
the genesis of the archaean rocks which has been put forward in this work.

The retardation of radiation at this stage by the nebulous envelope will be
seen to be very great, if we consider only the fact that owing to (a) the high-
temperature conditions of the surface of the barysphere as the lithosphere
began to form upon it, and (6) the enormously high temperature at which
steam undergoes complete dissociation into elemental hydrogen and oxygen,
most of the water now existing on the globe and shut up within the
lithosphere, must have then existed as aqueous vapour in the outer nebulous
envelope ( the primordial atmosphere). The effect of this in retarding radiation
has been rendered perfectly clear by the researches of Tyndall and others on
radiant heat; and in the lithosphere-forming stage its effect differed only
quantitatively from what we observe on a cloudy summer evening. Again, it
will be seen that the action of the non-diathermaneity of the primordial
nebulous atmosphere in shutting off a large portion of the solar rays does not
materially affect the ai'gument ; for this is quite independent of any such
accession of heat from sources outside the Earth's mass. And this
consideration is an essential one, since recent researches in astronomical
physics* show that we cannot safely assume that the central orb of our
present Solar System had at that time reached its present 'solar' phase.

Nor are we to imagine that deposition by condensation which was the main
factor in the formation of the lithosphere (at least in its earlier stages) went
on over the whole surface with geometrical regularity. On this point it will
suffice to quote from the concluding paragraph of Suess's 'Entstehung der
Alpen' : "Tacchini has shown that irregular areas are developed on the solar
disk which give only the magnesium-spectrum, and that these vanish again.

* See especially Prof. S. P. Langley, The New Astronomy, (Boston, 1888).

24 EOCK-METAMOBPHISM.

Zollner and before him Bullialdus have attributed the periodically-variable
strength of the light of certain stars of variable brilliancy to the presence of
dark fields of slag and of glowing 'Meers' upon the surface of their rotating
spheres during the process of cooling. The condition of our planet may well
have been once similar." But this carries us a step further, when, by the
continued operation of the same laws, not only did the intensity of heat reach
a maximum, followed by another phase of considerable duration, in which
developement of heat in the first-formed lithosphere and dissipation by
radiation (as the nebulous envelope resolved itself, owing to progressive
condensation, gradually into an atmosphere) were practically balanced (the
'solar phase'); and this again (with further advance towards atmospheric
conditions) was followed by an inversion of the relation that previously
subsisted between developement of heat and its dissipation by radiation, the
latter now getting the mastery, the cooling stage, through which our planet is
still passing, having set in.

There are reasons for believing that the larger mass of Jupiter is at present
in a much earlier phase of the cooling-stage than our Earth.

/?. Secondary Paramorphism. Eocks having been once
formed there is no cessation of the action of chemical and
physical laws, though the conditions under which they continue
to act are changed by dissipation of energy. We must
therefore be prepared to recognise a progressive order of
changes among the minerals of the same rocks, especially by
the action of percolating and interstitial water, and to some
extent, where within reach of the atmospheric waters of the
globe, by the action of carbonic acid and other atmospheric
gases dissolved in rain-water, as well as by the potent action
of the humus acids, which result from the decay of vegetation
(under conditions of incomplete oxidation), especially in the
dense jungle-growths of tropical regions."

Such agencies lead in many cases to the formation of a
series of minerals which were not formed in the original
genesis of the rock, and are therefore conveniently spoken of
as 'secondary minerals.' Further, inasmuch as the formation
of such secondary minerals formed as they are in the main
by derivation from the original and primary minerals of the
rock in no way (without concomitant hyperphoric change)
alters the composition of the rock as shown by a bulk-analysis,
we may include all such changes under the head of ' secondary
paramorphism.' This is exemplified in a great variety of rocks,
some instances of which may be here cited.

a. In the formation of secondary minerals in phyllites or
slates, and in shales, as separation-products from the original
materials of the rock (see Sect. iv.).

o. In the separation-out of the siliceous cementing-material
of altered and compacted volcanic tuffs.

c. In the decomposition of the felspar contained in some
varieties of sand, to furnish the siliceous cement (with its

* Strikingly exemplified in the formation of laterite, as an extreme product.
(See Appendix i.)

SECONDABY PABAMOKPHISM. 25

inclosed kaolin) of some quartzites. The case of the genesis
of the Sarsen-stones, which I have attempted elsewhere to
explain,* may be mentioned here. The theory which I put
forward five years ago has since been quoted by other writers,
and I have met with no attempt to refute it. The theory
itself was worked out on purely chemical lines and confirmed
by microscopic examination of thin sections.

d. In the deposit of calcite in igneous crystalline rocks by
the action of carbonated waters upon felspars containing lime
to which I drew attention in the paper just referred to.f

e. In the formation of secondary quartz out of the matrix
of acid rocks, such as the quartz-porphyries of the Bozen
district.

On general grounds there is no valid reason why in acid rocks secondary
SiOa should not be formed as freely as more basic minerals (chlorite, serpentine,
etc.) are formed in highly basic rocks.

No one can study Rammelsberg's 'Mineralchemie' without being impressed
with the very large proportion of ordinary rock-forming minerals which are
not in the strict sense of the word pure chemical compounds ; that is to say
compounds in which the metallic (basic) and non-metallic (acid) constituents
are present in exact equivalent proportions. In many cases of minerals formed
by dry heat (not a trace of water being contained in them), as the primordial
minerals must have been, the excess of one or the other class may have
promoted ' fluxing,' so that the minerals have assumed a definite individuality
on crystallizing from dry fusion ; but it by no means follows that in such cases
the most (chemically) stable compounds have crystallized-out. There must
remain in them therefore an innate tendency to reach the more stable
condition of chemical equivalency (especially when aided later by the access to
them of superheated H^O), and this could hardly take place without the
separation-out of the excess present (beyond the equivalent proportions) of the
basic or acid constituents as the case might be. Such paramorphic change
must clearly be distinguished from alterations resulting from chemical
reactions of foreign mineral-matters in aqueous solutions.

The origin of some of the quartz in quartz-porphyries still admits, I think,
of discussion (see Zirkel Besch. der Min. u. Gest. pp. 330, 331), especially that
which contain portions of the felsitic basis.

/. JuddJ has recently recorded the presence of many
secondary minerals as alteration products in the Tertiary
igneous rocks of the north-western portion of the British area.
That author recognises three agencies as brought into play in
this process of secondary paramorphism :

(i.) Solvents acting under pressure, producing that class
of change which he distinguishes as 'schilleri-
zation.'

* See Proc. Geol. Assoc. vol. viii. pp. 153-160; also Brit. Ass. Report
(Southport, 1883), p. 505.
t Ibid, p. 158.
t Q. J. G. S., vol. xlii, pp. 80-86.

26 KOCK-METAMOKPHISM.

(ii.) Steam and other gases acting upon lavas at the
surface. Among the results of this kind of action
he mentions palagonite, amygdaloidal in-fillings
of steain-cavities with such derived minerals as
zeolites, chlorophaeite, epidote, quartz, calcite ;
while secondary quartz and calcite are in some
cases "found to have been introduced into the
substance of the rock."

(iii.) Atmospheric agents producing kaolinization of the
felspars accompanied by hydration and recrystal-
lization of the silicate of alumina, and in some
cases concomitantly with the breaking up of the
felspar " the separation of considerable quantities
of silica which crystallizes as secondary quartz."
Varietal changes in the augites, paramorphic
conversion of augite into hornblende, conversion
of augite into viridite and this eventually into
hornblende, conversion of olivine into serpentine,
are among the most interesting examples of
secondary paramorphism noted as occurring in
those particular rocks.

Special interest attaches to the recent valuable work of
Professor Judd, which has so greatly enriched the Quarterly
Journal of the Geological Society. I do not think however
that the idea of ' petrological provinces ' (which he has
recently formulated) is altogether new to science ; for it seems
to underlie much that has been written by continental writers
and to be tacitly implied as a limitation of the views of some
of them as to a temporal order of succession of the eruptive
rocks. In a descriptive classification of the crystalline rocks
we must take them as they are presented to us in nature ;
and (the importance of time as a factor in all the complicated
mineral changes involved, together with the importance of
high temperature and pressure to increase the solvent action
of water being admitted in the more extreme changes) common
sense would lead us to expect to find generally those eruptive
rocks, in which paramorphic change has taken place in the
more extreme degrees, rather among the palaeozoic than among
the latter rocks.*

g. Among the workers in this country whose researches have thrown great
light on the mineral changes (secondary paramorphism) which take place in
crystalline (igneous) rocks, a very high place must be assigned to Allport,
not merely for the value of his papers, but as a recognition also in him of a
pioneer.

In his classical paper on the Rocks of the Cornish Peninsula (Q.J.G.S., vol.
xxxii) he says of the 'Altered Dolerites' that "microscopic examination

* cf. Bouney's Presidential Address, 1886.

SECONDARY PARAMORPHISM. 27

clearly shows that the pyroxenic minerals (augite and diallage) have frequently
been converted into a hornblendic substance, and the variety enstatite is found
filling cavities and fissures precisely in the same manner as other products of
alteration." (p. 424). " The alteration that has taken place appears to be the
result of internal (paramorphic) rather than of external action decomposition
and rearrangement (a vague term, requiring considerable definition from the
chemist's point of view) of mineral substances in situ and not to any great
extent to the introduction of new minerals from without."

On p. 425 occurs the inferential statement :

' ' Two rocks of similar origin and composition may follow different lines of
metamorphism according to the nature of the active agent and the duration of
its operation." As examples he points to the following :

(1) The Clickor Tor dolerite at a distance from the granite having been
altered into a serpentinous rock by aqueous agencies, the serpentine forming
psaudomorphs after olivine, and the rock having been originally an olivine-
dolerite ;

(2) The Penzance dolerites in close proximity to the granite having been
transformed into hornblendic rocks, varying (inter se) in texture and extent of
alteration according to the coarseness or fineness of the original crystallization.

In the same paper it is shown that "many of the metamorphic rocks
described have undergone a second series of changes, as indicated by the
occurrence of micaceous and chloritic pseudomorphs after tourmaline, and an
alteration of the mica ; " that "the granite has also suffered similar changes ; "
and that among the more basic rocks hornblende-schists may be metamorphosed
igneous rocks, some being derived from dolerites and gabbros (implying both
secondary paramorphism and metataxis), while others are very probably
foliated diorites (implying metataxis only).

In his great paper on British Carboniferous Dolerites (Q. J.G.S., vol. xxx)
the same author has shown how great and varied are the changes which these
have undergone, including such as

(1) Devitrification of an original glassy ground-mass.

(2) Partial kaolinization of felspars.

(3) Formation of magnetite grains within augite-crystals.

(4) Partial conversion of augite into 'a grey fibrous or granular substance.'

(5) Conversion of augite into 'a fibrous hornblende.'

(6) Alteration of olivine into a green substance, generally serpentine

chlorite, chlorophaeite, or ferrous silicate.

(7) Further alteration of serpentine into haematite.

(8) Pseudomorphs after olivine composed of calcite and serpentine and other

minerals.

(9) Aggregations of grains of augite in felspar.

Allport suggests (p. 540) the "long-continued action of mineral-waters" (i.e.
waters holding mineral salts in solution) and circulating in the interior of the
1 crust ' as the cause of the changes which occur in olivine.

A comparison of these changes with those described by Judd in the Tertiary
igneous rocks and referred to, (supra, p. 26,) brings out some differences in the
action of water at or near the surface and at depths in the crust.

The differences observable in parts of the same rock in the same quarry
(partly from unequal admixture of the original minerals, and partly from
different degrees of paramorphic change which they have suffered) are seen to
be as great as those by which such rocks as dolerite, basalt, anamesite, aphanite,
are distinguished ; so that there would seem to be little reason why all the
basic augitic rocks should not (as Allport argues) be included in the one group
of dolerites.

In subsequent papers, (1) 'On the Ancient devitrified Pitchstones and
Perlites from the Lower Silurian District of Shropshire' (Q.J.G.S., vol. xxxiii)
in which he compared the changes which these have undergone (as exhibited in
the Lea Rock of the Wrekin, which I had the pleasure of examining a few
years ago in the company of Dr. Callaway) with the changes exhibited by the

28 KOCK-METAMOBPHISM.

later pitchstones of Meissen and perlites from Krenmitz and Schenmitz ;
(2) 'On the Diorites of the Warwickshire Coalfield' (Q.J.G.S., vol. xxxv),
Allport has shown how little essential difference there is in rocks of different
ages of these two several classes or groups, after we have eliminated what
would be included (in the nomenclature of this work) under the heads of
(a) metataxic, (6) hyperphoric change, the latter being determined mainly by
two circumstances in the environment of the rocks, (i) the nature of the adja-
cent rocks, (ii) extent and time of exposure to the atmosphere (chiefly resulting
in the formation of haematite by the introduction of free oxygen, and calcite by
the introduction of CC>2 held in solution in atmospheric waters), amounting in
fact to what is commonly called ' weathering. ' Leaving therefore these minor
and localized alterations out of account, may we not generalize from Allport's
work so far as to regard the really paramorphic changes which the igneous rocks
exhibit as consisting in the aggregate (from the chemical point of view) in the
mineral-transformation from those states of combination which were most
stable in a condition of dry fusion to those which are most stable at lower
temperatures in the presence of water ?

To Allport seems then to belong the great merit of having laid the foundation
of a natural system of classification of the igneous rocks. That such a system, on
such principles as are illustrated in the outline suggested in Bonney's Presi-
dential Address (1885), must ultimately supersede more artificial systems (as in
Botany the 'Natural Orders' of De Candollehave superseded those of Linnaeus)
seems hardly doubtful.

The masterly researches of Allport (infra) followed up by those of Judd,
Bonney and others, have made out such a strong case for the genetic identity
of the Palaeozoic and Tertiary rocks of each class or family, that it is with
great satisfaction that one finds so high an authority as Credner (Geologic, 6th
ed. 1887, p. 303), after doing full justice to the systems of Zirkel and
Rosenbusch and drawing a parallel between the several types of the
"palaovulkanische und neovulkanische Gesteine," admitting that "zwischen
jenen alteren Eruptivgesteinen und den vulkanischen Producten der Jetzzeit
ist mit Bezug auf ihre Entstelmngsweise nur ein zeitlicher Unterschied."

What is an individual rock ? This question must be answered with reference
to architectonic functions displayed rather than to composition. Here we look
for the great broad class-characters. But when we come to break up a class
(such as (a) massive crystalline rocks, (b) phyllolithic crystalline rocks, (c)
clastic rocks,) into sub-classes, or groups, or families, we must rely upon
characters which are subordinate to those which have determined our classes.
It is here that the quaestio vexata crops up. Are we (following Rosenbusch)
to make " mode of occurrence in the field " the main basis of our sub-classes
and orders or families (which is rather suggestive of the adoption of a
grouping into forest trees, shrubs and herbs, as a basis for classification within
the limits of either of the great classes Exogens, Endogens, or Acrogens) ; or,
are we (as Bonney suggests) to give the first place to the broad differences of
mineral composition (which is more analogous to the grouping of e.g., the
Dicotyledons into sub-classes by floral structure, as is usually done in the
natural system)?

Look at it how you will, there are considerable difficulties for the igneous
rocks in considering anything less than the entire crystalline mineral complex,
which is the product of any particular centre of igneous activity, as possessing
definite individuality. This is well shown by the well-worked-out ' Eruptiv-
stock ' of Predazzo, as sections of it are exposed by erosion in the flanks of the
deep gorges of the Avisio and the Travignolo, (Credner, op. cit., p. 587).
Here tourmaline-granite, syenite, melaphyre, augite-porphyry, orthoclase-
porphyry, are all found with their respective morphological modifications
some occurring as plutonic (Tiefengesteine), others as dykes (Ganggesteine,)
others again as out-poured lavas, (Ergussgesteine) yet with no more definite
individuality than can be assigned severally to the root, the stem, and the
foliage of an individual tree. We should scarcely attach any serious value to

SECONDARY PARAMOBPHISM. 2-9

a classification based on the morphological characters of separate collections
respectively of chips or fragments of roots, stems, and leaves of plants. It is
the way in which their structural characters are blended together in the
individual plant that serves as the starting point in the natural system of
classification in the Vegetable kingdom. Such an extension of the term
'individual' would absorb the troublesome factor of 'temporal succession,'
and would do no violence to the idea of ' petrological provinces.'

h. In the case of interbedded eruptive rocks which have
heen formed as sub-marine lava-flows, it is clear that the
secondary paramorphic changes which they must have under-
gone in the process of cooling will be somewhat different
from those which have taken place in sub-aerial lava-flows.
In the former case the salts contained in solution in marine
waters would set up a rather different set of reactions within
the rock mass. The salts of the alkali-metals contained in
such waters would perhaps not undergo much change by
contact with the minerals contained in the original lava-flows,
since both their acids and bases are electro-chemically
stronger than those contained for the most part in the
silicates. In the comparatively rare instances in which
reactions were set up between them (e.g. in the reaction
between silicate of potash contained in a felspar or mica and
sodium chloride) the K Cl thus formed would be removed in
solution, while the nature of the felspar would be altered by
the substitution of sodium for potassium ; and in this way it
is conceivable that in many sub-marine lava-flows orthoclase
may have been converted into albite.* In the case of the
salts (the haloids, the sulphates, chiefly) of lime and magnesia
we should expect changes of this kind to occur more exten-
sively. These would all react upon silicates of the alkalies to
form corresponding soluble salts of those metals, the Ca and
the Mg replacing the Na or the K to form non-soluble silicates.
It is impossible therefore to deny that serpentinization and
other essentially similar changes may have taken place in this
way, or that orthoclase and albite may have been converted
by such a process of sub-marine paramorphism into oligoclase,
anorthite, labradorite, or saussurite.

The extent to which water in such cases might penetrate the glowing molten
mass would vary with the pressure as determined by depth below the surface ;
and perhaps in any case steam only would penetrate the mass under such
conditions. But there can hardly be any doubt that as the mass cooled it
would be penetrated by saline water. The idea seems worthy of consideration
in working out the history of such rocks as Troktolites and Eucrites, and
generally of those rocks in which anorthite occurs. See (e.g.) Teall (Q.J.G.S.,
vol. xl, pp. 234-235.) The two chief varietal characters noted by Mr. Teall in

* For a probable instance of this in a dyke penetrating the strata at the
bed of a Cambrian sea, see Q.J.G.S., vol. xliv, p. 410.

30 ROCK-METAMORPHISM.

the well-known Tynemouth Dyke, are (1) porphyritic crystals of anorthite
(' very irregular, rather crystalline aggregates than simple crystals'), (2) small
spherical amygdaloids of calcite. The very unequal distribution of these in
the rock, and its position in the Carboniferous rocks beneath the magnesian
limestone, point to the action of infiltrating water of the sea which
yielded the magnesian limestone strata.

May not the richness in magnesian-silicates of some altered and compacted
diabase-tuffs be possibly accounted for in some instances by the infiltration of
marine waters where the volcanic ejecta were spread out on a submarine floor ?
(See Kalkowsky Lithologie, p. 120).

From this point of view especial interest seems to me to attach to the
exhaustive study Judd has made of the ' Tertiary and Older Peridotites of
Scotland' (Q.J.G.S., vol. xli, pp. 354 et seq.). Most instructive are the
alterations noted in the felspars (plate x) ; and the occurrence of sulphates and
chlorides in the liquid enclosures of some of these (op. cit., p. 376) is extremely
suggestive of the action of infiltrating saline waters. On p. 377 I find that
Judd notes " serpentinous and other decomposition products " as results of the
" passage through crystals of felspar of water from the surface." Kaolinization
is certainly the general result of the action of carbonated atmospheric waters ;
but in all such cases we have only the action of a free acid ( CO-2 ). In saline
waters (whether furnished from the sea or from underground circulation of
mineral- waters) the case is quite different : the several bases, with which the
acids are combined in the dissolved salts, must be accounted for ; and from the
chemical standpoint we can only account for them as has been suggested (by
the formation < f silicates with bases of lower chemical energy), unless they
remain mechanically mixed with the quartz which results from the SiOa
replaced by the stronger acid. (See A pp. ii, Notes S.) On general grounds
the latter would appear very unlikely in cases of the kind here discussed.

We are justified from such considerations in declining to
accept the evidence of microscopic and chemical analysis as of
itself conclusive, without reference to evidence in the field
as to the history of such rocks. By similar reactions we
can understand conversion of potash-micas (muscovite and
damourite) into soda-mica (paragonite) by the action of salts
(chiefly the haloids) of sodium ; and the ultimate production
of magnesia-mica (containing in some cases more than 30 per
cent, of MgO), the iron-micas, and even baryta- and lime-mica
(margarite). For data for these applications of chemical
principles see Kammelsberg.* The same high authority tells
us (p. 510) that " viele Glimmer sind offenbar Umwandlungs-
producte alterer Silicate und deshalb leicht Gemenge."t

i. An interesting case has recently been described of alleged
regional metamorphisni of vast areas of sedimentary rocks of
Cretaceous age in America (see Nature, May 27th, 1886). It
is not at all impossible that the chemical agencies, which have
been during all time aod are still at work, have been able to

* Mineralchemie., pp. 510-537. See also Roth's Allyem. u. Ch. Geol.
Bd. I., p. 68.

t An instance of the derivation of mica from felspar has recently been
described by Bonney (Q.J.G.S., vol. xliv., p. 36, fig. 2) ; and Kalkowsky
(op. cit., p. 208) remarks that the felspars produce by their decomposition
kaolin and micaceous minerals (glimmeraitige Mineralien.)

SECONDARY PARAMORPHISM. 31

produce in some regions a certain degree of paramorphic
change, which is however quite distinguishable with modern
methods of research from the characteristic crystalline struc-
ture of the archaean rocks. The derivative relation of these
clastic rocks to the granite in the Californian region is similar
to that of the Planer Sandstein of the Dresden country and
Saxon Switzerland to the syenitic massif of that region.

On many points connected with the Californian region
described by Mr. Becker, we must for some time suspend our
judgment.

The prevalence of zoisite may very likely point to an
extensive conversion of the carbonate of lime (which forms the
chief cementing material of these clastic rocks) directly into
silicate of lime, by the replacement at high temperature of its
C0 2 by the Si0. 2 of the quartz ; it being an experimental fact
well known in the chemical laboratory that crystalline
quartz in a state of fine division can thus form silicates of the
alkalies by simple contact with their carbonates in a state of
fusion at ordinary atmospheric pressure ; and so probably can
form silicates of the alkaline earths" in a state of dry fusion,
such as that in which statuary marble must have once existed.
The silicate of lime may have served as the basis from which
other minerals have been formed, and so the peculiar "pin-
cushion " arrangement of needle-like crystals stuck in the
quartz-grains may be partly accounted for.

So far as the evidence at present to hand seems to carry us,
it looks as if the case cited here were one of slight secondary
paramorphism extending through an extensive, region. To
assume however that this is but an incipient stage of a
metamorphic process, whereby the rocks of this region may
ultimately be converted into crystalline schists or gneiss is
altogether unwarrantable. f

k. Of the "resolution of clastic grains into crystalline
aggregates " with the simultaneous conversion of sandstone

* This deduction has been experimentally verified (see App. ii. Note A.)
+ Mr. Becker has dealt with the rocks of this region in his Essay which
appears in the 'Etudes sur les ScMstes Crystallins' published by the Inter-
national Geological Congress, 1888 ; but it can scarcely be said that much new
light has been thrown upon the subject. The crucial point of his argument
is the alleged transition from the great crystalline series (which have been
apparently and to a subordinate degree rendered schistose along crush-planes)
covering 3000 square miles and more into the clastic series, which have under-
gone excessive crushing. Such alleged transitions have been disproved over
and over again in Europe, and we want more precise information as to the
nature of the crushing and welding that has occurred at their junction.

Extensive secondary developement of crystalline minerals in fissures, even
in a rock which has been in part ' reduced to a mere rubble,' with an elevation
of temperature corresponding to the intensity of the crushing forces, can
scarcely be said to have produced a crystalline schist, when the ' unfissured
masses of considerable size never show any notable alteration.' (p. 111.)

32 ROCK-METAMORPHISM.

into quartzite I have myself observed a local instance in the
metatropic alteration of the basement-beds of the Keuper
sandstones of Grinshill, Salop, where they are faulted (as seen
in the lane leading from that place to Clive) at a high angle
against the Bunter Sandstone. The Bunter Sandstone is
altered against the fault into a sort of brick, and the alteration
of the Keuper Sandstone into a quartzite extends from twelve to
fourteen feet into the Keuper beds, beyond which these assume
their normal character. There is a developement of quite
large macroscopic crystals of a flesh-coloured silicate along
certain zones of the rock near the fault. The phenomena
presented in this faulted section seem to me to afford a very
good example of the effects of the intensity of heat when
suddenly developed by pressure and sliding friction sufficiently
concentrated, as in some of the late Mr. Mallet's experiments.
It seems reasonable to regard such occurrences as concomitant
subterranean phenomena of great earthquakes, and as being
no more related to such so-called ' regional metamorphism '
as is seen in the 'crystalline schists' of the archaean rocks
than a sprat is to an elephant or a whale. (See App. ii.)

In nature no limit can be recognized in the operation of
those changes to which we have assigned the name ' Secondary
Paramorphism,' in effecting which water is the chief agency.
They are seen in operation not only in rocks commonly called
' metamorphic,' but in igneous (plutonic and volcanic) and
sedimentary rocks also. Secondary quartz crystals (as was
shown by the late J. A. Phillips, and cases of which I have
myself recorded*) are developed on sand-grains; quartz crystals
and intergrowths of crystals of quarts and felspar are found
developed on the surfaces of blocks of felspathic rocks in some
of the older conglomerates, as in those in Euba near Chemnitz
quoted by Crednert; crystalline granular aggregates of quartz,
orthoclase, oligoclase, mica, and tourmaline are found deposited
by mineral waters in fissures ; veins are formed in granitoid
rocks by the infilling of fissures with secondary minerals
dissolved out of the rock-mass, as in the granulitic rocks of
Saxony, in the Eiesengebirge, in the Isle of Elba, in the gneiss
of North America (Credner, op. cit. and the authors there cited.)
Deposition of minerals in the drusy cavities of rocks, from
which those minerals have been derived, are further examples
of this, one of the most universal of nature's operations. J

* Geol. Mag., dec. ii, vol. X., p. 412.

t 'Elemente der Geologic,' 3rd ed., p. 203.

J To these we may add the cases of secondary change recently described by
Bonney (Q.J.G.S., February, 1888,) (i) in the matrix of the Obermittweida
conglomerate, (ii) in the matrix of the conglomerate of Sudbury, Canada.

SECONDAEY PAEAMOEPHISM. 33

To sum up, in the words of the writer just quoted, "The
tendency of water universally is either to dissolve the mineral-
constituents of a rock directly, or, after decomposition of
insoluble compounds, to remove at least some portion of them
in solution."*

Of the changes considered here under the head of ' Secondary Paramor-
phism ' it may be said generally, that they are local, partial, and accidental ;
they result in giving varietal differences to different portions of great rock
masses, but they are in no way essentially connected with those characters
which give to any great rock-mass a definite individuality, as performing a
determined function in the developement of the earth's lithosphere. As Judd
cogently remarks, (Q.J.G.S., vol. xli, p. 362), "much confusion has been
introduced into petrographical literature in consequence of all the characters
presented by minerals being treated as if they had precisely the same [degree
of] significance. While some of the characters of the rock-forming minerals
are original and essential, others are, as certainly, secondary and accidental.
The minerals, since their first crystallization, may have undergone several
series of changes totally dissimilar in kind, and resulting from causes
altogether different." The italics in this quotation are mine.

iii. METATEOPY.

In using this term, we must give ourselves rather more
latitude than is allowed in the use of the sister-word ' allotropy '
in chemistry. Strictly, allotropy implies the assumption under
different physical conditions of a different set of physical
properties by one and the same chemical body; and its use is
generally confined to the elementary bodies. Here we shall
have to preserve the main idea, and, while extending the term
metatropy to chemical compounds, and even to mixtures of
them in some instances, include under it only those instances
in which essential alteration takes place in the physical
character (T^OTTOS) of the rock (hardness, crystalline form,
cleavage, fracture, optical properties, conductivity, specific
gravity, and so on), while no essential change occurs in its
chemical composition. Such changes, it will be seen, may be
considered without reference to the original genesis of the
rock. The conversion of seam-coal into anthracite (as in the
Culm of the Alps and Germany), and that in some cases into
graphite, are changes in the physical character of rock-masses
which may be designated ' metatropic,' concomitant with other
changes in th'e rocks among which they are interstratified.
In this case we have a slight change in chemical composition,
with a corresponding increase in the percentage of the elemen-
tary carbon, but the chemical composition remains essentially
the same.

* Roth has discussed the whole thing in his usually masterly way in his
Allgemeine und Chemische Geologic. To his laborious collection of facts as
chemical data for the study of such phenomena, references are made in the
sequel of this work.

34 ROCK-METAMOKPHISM.

The formation of palagonite and hydrotachylite out of the
glass of basalts by hydration, the change of anhydrite into
gypsum by taking up water of crystallization, may perhaps be
considered cases of metatropy, as also the conversion of
arragonite into calcite by dry heat, and the opposite change of
calcite into arragonite by solution and reprecipitation at a
higher temperature. Such instances of ' contact metamor-
phism ' as the conversion by heat of the finer grauwackes
and shales into hornstone or porcellanite, the conversion of
coal into coke, the conversion of sandstone into quartzite,
and the conversion of limestone into marble, are all instances
of metatropy. Occasionally there would appear to be a slight
and subordinate metataxic change also, resulting in the ap-
pearance of small inclusions of earthy or marly matter in
crystalline marble, unequal distribution of the oxides of the
heavy metals, a linear arrangement of authigenous mica in
cipollino (as is well seen in the weathered columns of the
Eoman Forum) the cleavage of some Alpine marbles. In
some of these changes heat has been the sole or principal
agent ; but in the case of marble pressure also was absolutely
essential to prevent the destruction of the carbonate of
lime by dissociation into calcium oxide and carbonic acid
gas.

Polymorphism, a phenomenon exhibited by many minerals,
often to the extent of rock-masses of considerable magnitude,
may be included under the head of metatropy. Here again
the study of the mineral constituents of rocks on the chemical
side seems to help us, as the facts next to be cited plainly
show.

(1) Temperature may affect the crystalline form of the
mineral, as illustrated in the dimorphism of carbonate of
lime, to which reference has been made before.

(2) The crystalline form of the same essential compound
may vary with the amount of molecular water which its
molecule holds in combination, as in the case of carbonate of
magnesia, which crystallizes out of an aqueous solution of the
salt in carbonated water, when the solution is left to stand in
the air, in the following forms :

(a) in monoclinic plates with the composition Mg CO 3 +

5 H 2 in the cold of winter ;

(b) in a mixture of monoclinic prisms with the com-

position Mg CO 3 + 4 H 2 and nests and balls
of rhombic twinned crystals of the composition
Mg CO 8 + 3 H 2 at ordinary temperature ;

Mg CO 3 + H 2 O from tepid water.*

* W'islicenus, Anorg. Chem., 600.

METATBOPY. 35

With the case of the variation of molecular structure along with differences
in the proportions of molecular water presented by magnesium carbonate, as
observed in the chemical laboratory, may be mentioned the case of the native
hydrates of alumina:

Hydrargillite, Al 2 3 + 3 H 2 [H 6 A1 2 6 ] chalcedonic.

Bauxite, A1 2 O 3 + 2 H 2 O [H 4 A1 2 O 5 ], earthy.

Diaspore, A1 2 O3 + H 2 O [H 2 A1 2 04], highly crystalline (isomorphous with
chrysoberyll (Be A1 2 O 4 ).

The most crystalline form here, it will be noted, has least water, while in
the Mg C0 3 series it has the highest proportion of water,

In the silicates of copper again we have

Dioptase, Cu Si O 3 + H 2 O, crystalline:
Kieselmalachit, CuSi 3 + 2H 2 0, non-crystalline.

(3) The presence of an accessory^ mineral may influence
crystalline form. In the crystallization of carbonate of lime
the presence of small quantities of the carbonates of barium
and strontium (for example) increases greatly the proportion of
arragonite, with which those minerals are isomorphous ; a fact
which would suggest that the normal crystalline form of the
carbonates of the alkaline earths is rather that of the rhombic
prisms of arragonite than the hexagonal system, to which the
many crystalline forms of calcite may be referred.

Recent researches seem to suggest an allotropic relation between certain
augites and certain hornblendes. Within the wide range of variation which
these minerals severally display in their chemical composition, it is not at all
unlikely that for certain percentage-compositions, the same mixture may
crystallize under certain physical conditions as augite, and under other
physical conditions as hornblende. But have we any right to generalise to
the extent of asserting that any augite may undergo a metatropic change into
hornblende ?

A negative answer to this question is suggested by the following average
percentages :

Augite Hornblende

(29 analyses). (24 analyses).

Si 2 4970 44-11

A1 2 O 3 578 10-86

Fe O 8-58 7'61

Fe 2 O 3 -85 6-13

Ca O 2071 12-35

Na 2 O -19 1-67

K 2 O -07 1-41

MgO 9-95 14-20

from analyses given by Rammelsberg.

Rammelsberg (op. cit. pp. 411, 414) regards both minerals as ''isomorphous
mixtures of silicates " with the sequi-oxides Fe 2 O 3 and A1 2 O 3 , though his
earlier notion that the A1 2 O 3 was present in hornblende as aluminate, may not
be altogether untrue ; and, if so, this would possibly help to determine the
crystalline form of hornblende. More importance perhaps is to be attached to
the frequent occurrence in the aluminous hornblendes of such accessory
components as TiO2, Cr0 3 , and F, as influencing crystalline form. (Ibid, pp.
416-418.)-

Again, Epidote and Zoisite are almost allotropic forms.

In this connection may be mentioned those curious globular
concretionary structures with a radiating prismatic sub-
crystalline texture suggestive to a chemist's mind of crystal-

36 ROCK-METAMOKPHISM.

lization out of supersaturated solutions which abound in the
Magnesian Limestones of Durham, occurring sometimes nearly
a foot in diameter, while smaller ones often make up the entire
rock-mass of whole beds of limestone, containing 98 per cent,
of carbonate of lime.*

Of all the changes which we should call metatropic those of
vitrification and devitrification of rock (as, e.g., obsidian and
tachylite) are by far the most complicated. It is pretty
generally recognized that the former of these is the result of
solidification by rapid cooling. Much obscurity however still
hangs over the latter phenomenon ; but perhaps the con-
sideration of a few known chemical facts may go a little way
towards penetrating it, and will lead us to recognise a change
of molecular structure as the essential part of the process of
devitrification ; though the physicist and the chemist pronounce
with one voice our ignorance of the actual constitution of the
molecules of solid bodies. Two of the best-known instances
of allotropy are those of the elements sulphur and phosphorus. 1 ^

Both these bodies may undergo such a metatropic change as
to assume an allotropic modification, which we are justified in
calling the 'vitreous condition.' I have for some years used
this term with reference to them, and observe that this use of
it is gradually finding its way into text-books of chemistry.
In both cases the vitreous condition is induced by rapid cooling ;
common or vitreous phosphorus, by the arrangement employed
for casting it into sticks ; sulphur, when poured in a molten
state at temperatures not far below its boiling point into cold
water. The translucent flexible needle-shaped prisms which
are formed when sulphur solidifies in the dry way at the
higher temperature of its fusing point (115C) must be regarded
as a vitreous form, so far as their internal texture goes, since
they are quite isotropic.J That in the vitreous condition of
both these bodies the molecular structure is not the most
permanent or stable appears from several considerations.
Vitreous sulphur (both in the plastic and in the prismatic state)
is known to assume the crystalline state in forms of the
ortho-rhombic system in a few days, the translucent mass
becoming opaque (devitrified) as the result of crystallization.

* They must not be confounded with concretionary dolomitic ' mudstones '
found in other parts of the same rocks, on the weathered surface of which a
laminated structure can be easily traced. In the summer of 1886, to the
surprise of Mr. Howse, of the Newcastle Museum, I split some of these with
the hammer, and disclosed casts of Axinus within them.

"t* Carbon, another allotropic element, is omitted here, as it is not known in
the glassy or isotropic modification ; but we do know that in its crystalline
state (Diamond, Sp. Gr. 3 '5) its density is greater than in the amorphous state
(Charcoal, Sp. Gr. 1-82) (ef. Note L, App. ii.).

They are in fact crystallites,

METATEOPY. 37

Further there is found both in flowers of sulphur and in vitreous
sulphur an amorphous electro-positive form of the element
(insoluble in CS 2 and other solvents in which normal crystal-
line sulphur readily dissolves), and this amorphous sulphur is
found in greater proportion as the sulphur has been more
strongly and continuously heated and more suddenly cooled.
This may be well compared with the amorphous ' dirt ' which
the microscope reveals in the vitreous forms of volcanic rocks ;
so that this may perhaps be regarded as a concomitant product
of vitrification. Amorphous insoluble sulphur is also formed
on the surface of the mass when molten sulphur is allowed to
congeal in the presence of strong sunlight, just as vitreous
phosphorus is partly converted (with the assumption of various
shades of red) into amorphous phosphorus by the same
physical agent. Prolonged action of heat too, as is well
known, converts the whole of a mass of vitreous phosphorus
in the red amorphous variety ; while the same result is
effected by heat in much less time, if a trace of iodine or
phosphoric iodide is present. Amorphous sulphur however
ultimately assumes the normal crystalline form, as does also
vitreous sulphur ; and the change is accompanied in both cases
by the liberation of heat, the sudden crystallization of vitreous
(plastic) sulphur being accompanied by a sudden rise of
temperature from 93C to 110C, when the experiment is
properly conducted. (See App. i, a.)

We may say then with Prof. Wislicenus* that "the ortho-
rhombic modification of sulphur is at ordinary temperatures
the most stable, into which all others pass spontaneously ;
native sulphur occurs accordingly without exception in this
form." Selenium undergoes a similar series of metatropic
changes. Less is known about the crystallization of phos-
phorus, but vitreous phosphorus is well known to form a
crystalline sublimate, when kept for some time in the dark in
an exhausted and hermetically-sealed glass tube. In my
own work too I have obtained evidence of the spontaneous
transformation of the other allotropic forms of phosphorus
into a crystalline sublimate. We may then regard the
crystalline condition of both sulphur and phosphorus as the
most stable, as that towards which there is a constant strain or
struggle in both the other two molecular states. f This comes
out too when we consider their specific gravities ; both these
bodies have the lowest specific gravity in the vitreous, and the

* ' Lehrbuch der Anorganischen Chemie,' 236.

f See Appendix i. Such a molecular strain as is here suggested may, it
seems, occur and produce its optical effect in the mineral, altogether indepen-
dently of any incipient deformation due to external pressure ; an important
fact to bear in mind in microscopic petrology.

38 BOCK-METAMOBPHISM.

highest specific gravity in the crystalline state.* The thermal
phenomena point the same way. The rapid transition of
sulphur from the vitreous to the permanent crystalline state is
accompanied with liberation of heat as has been noticed
before ; and the heat of combustion of vitreous phosphorus is
known to be considerably greater than that of the other
modifications of the element, being about one-eighth greater
than that of the red amorphous variety. f Both these facts
seem to point to such a thing as latent heat of vitrification,
which will be seen at once to be an important factor in
devitrification generally. The work done by it seems to consist
in keeping down the molecular architecture of the body to a
simple primitive form ; more highly complex and more per-
manent molecules being built up when the latent heat is set
free.

We need not confine our attention to the consideration of
known metatropic facts in elementary bodies, since a vitreous
condition is assumed under certain physical conditions by such
compound bodies as arsenious oxide (As 2 O 3 ), metaphosphoric
acid (HPO 8 ), silica (Si 2 ), and borax (Na 2 B 4 O 7 + 10 H 2 0).
We must glance briefly at these.

(1.) Arsenious oxide in the vitreous or glassy state is well
known, and is produced in the refining process by the conden-
sation of the vapour upon the upper heated zones of the iron
retort which is used in the process. In time this glassy mass
becomes turbid and porcelain- like, as the crystalline texture is
developed within it. This assumption of the crystalline form
by glassy arsenic may be made to take place so rapidly by the
application of a drop of HC1 to a saturated solution of glassy
arsenic as to develope a beautiful phosphorescent light, through
the rapid liberation of the latent heat of vitrification. The
quantity of heat set free by one equivalent of arsenic (As 2 3 =
198) has been found to amount to as much as 2,652 calories in
passing from the vitreous to the crystalline state. J

(2.) Vitreous phosphoric acid (naetaphosphate, H P0 8 ).
This body is best obtained as a transparent solid isotropic mass
("glasige Phosphorsaure '") by calcining the ortho-acid in a
platinum crucible, and expelling in this way two out of the
three equivalents of the water of constitution of the ortho-acid.

Vit. Cryst.

* Sulphur, 1-98 .... 2.07

Phosphorus 1'83 . . . . 2.34

t Wislicenus. op. cit., 285 (1 gin. of red P. produces 5,592, 1 giu. of
vitreous P. about 6,400 calories).

J See Wislicenus, op. cit. ( 319.) "Beim Uebergange der krystallinischen
in die amorphe Modification wird namlich Warme latent, welche bei der
entgegengesetzten Umwandlung in Form von Licht und Warme \vieder frei
wird. Diese Warmemenge betracht auf die der Formel As2 63 entsprechende
Menge (198 Theile) 2,652 Warmeeinheiten."

METATBOPY. 39

Its chief interest for our present purpose lies in the fact that
it loses its vitreous condition by absorption of water and
changes into the ortho-phosphate, which if dried at 150C
crystallizes slowly in a dry atmosphere as rhombic prisms.
Every laboratory-worker is familiar with the fact that sodiurn-
metaphosphate is obtained as a transparent glass when
microcosmic salt is calcined. The most interesting fact about
the metaphosphates in connection with our present subject is
their polymerism. Under favourable conditions the meta-
phosphates are known to assume as many as five different
molecular structures, several of which crystallize. This
happens :

(a) with dimetaphosphates of the alkali metals ;

(b) with trimetaphosphates, particularly those of lead

and silver :

formed when fused sodium metaphosphate is
allowed to cool slowly. (Roscoe and Schor-
lemmer).

These facts afford a striking confirmation of our previous
induction from the behaviour of phosphorus and sulphur, as
to the formation of higher and more complex molecules in the
process of devitrification.

(3) Silica (Si O 2 ). Quartz or rock-crystal has a specific
gravity 2*6 and is probably in all cases formed out of aqueous
solutions* (Wislicenus, Anor. Ch., 375.) This fact may be
compared with the fact that both sulphur and phosphorus
crystallize out of solutions in the permanent crystalline form.
Glassy silica moreover obtained by melting any of its crystal-
line forms in the oxyhydrogen flame has a specific gravity of
only 2-2, the specific gravities of the intermediate forms come
between this and that of quartz (tridymite 2-3, asmanite 2-24).
These facts point also to a more stable and consolidated
molecular architecture in quartz than in any other modification
of the body. In glassy silica therefore we might expect an
internal strain or tendency to build up the more stable
molecules of quartz. I am not aware if this change has been
actually observed.! I have however been fortunate enough to
obtain the glassy form of silica in the wet way ; that is to say,
by leaving the well-known colloid hydrated silica, which was
precipitated by just neutralising a solution of sodium silicate
(' soluble glass ') with HC1, to stand for a long time in contact

* The direct product of combustion of silicon is probably the rare fibrous
mineral known as " amianthus " occasionally found in blast furnaces. (See Dr.
Percy's Presidential Address to the Iron and Steel Institute, 1886.)

t The late John Arthur Phillips's paper (Q.J.G.S., vol. xxxv.) 'On the
History of Mineral veins ' is well worthy of study from this point of view.

40 BOCK-METAMOBPHISM.

with water, which gradually evaporated away in the air of the
laboratory. When first formed it was clear and transparent
with the definite fracture of glass ; but in course of time it has
lost to a great extent its transparency, and at the edges the
fragments have become quite opaque. Prof. Bonney, to whom
I gave two or three small fragments of the substance, suggested
loss of water as the possible cause, and this turns out to be
the case.*

The production of tridymite, apparently by the liberation of Si O 2 from the
silicate of alumina contained in fire-clay by ZnO (Zinc-spinell being formed,)
has been observed in the walls of the muffels used in reducing ZnO to the
metal at Freiberg, (infra) The aggregation of silica into crystals in the
midst of a mass of kaolin has probably occurred in the china-clay pits of
Tintagel, and elsewhere in the Cornish Peninsula. I have some beautiful
specimens of these which were given me by the late John Arthur Phillips, the
crystals being perfectly bi-pyrarnidal.

(4) Borax. In the ordinary use of borax on the platinum
wire the crystalline salt is converted into a bead of glass by
the expulsion of its water of crystallization by heat. Its affinity
for water however is so great that in moist air it takes up
water again and resumes the crystalline form, the mass at
first becoming turbid and ultimately falling to pieces. f It can
assume two crystalline forms according to temperature:
Na 2 B 4 O 7 + 10 H 2 in monoclinic prisms from cold water,
and Na 2 B 4 7 + 5 H 2 in octahedra out of water above 60C.

(5) Calcium fluoride affords such a marked illustration of metatropic change,
that it ought to be added to the cases there cited. It occurs native only in the
crystalline form, either cubic or octahedral.

It can be artificially prepared :

(a.) As a gelatinous opalescent mass it is formed by the action of a solution
of K F or Na F upon Ca C1 2 in solution.

(b. ) As an amorphous powder it can be obtained by the decomposition of
Ca COs by aqueous H F. The fact that this amorphous Ca F 2 can be
converted into octahedral crystals by heating it to 250 C C in a closed tube with
very dilute H Cl is a noteworthy instance of metatropy.

(c.) As regular 8-hedral crystals it may be prepared (as well as Sr F 2 and
Ba F 2 ) by melting the carbonate with Na Cl + Na F.

In (b) the tendency of hydrostatic pressure to promote crystallization is
strikingly illustrated.

In the case of vitrification of As 2 Os the temperature requires to be just
high enough to prevent crystallization. Experiment has shown that this
requires a little over 200C if the temperature is maintained a fairly long
time. Vitreous As 2 Og can be produced more quickly by heating the substance
under the pressure of its own vapour in a sealed tube. At rather lower
temperature the rhomboedral mineral Claudetite is occasionally formed.
Bodies which, like As 2 Os and ammonium salts, sublime under ordinary pressure
seem to lend themselves to the investigation of the solid-liquid 'critical state.,

* See Appendix i.

f Devitrification may be seen even macroscopically in a few hours, when a
borax bead : ' is exposed to ordinary atmospheric moisture.

METATEOPY. 41

From the facts which we have now reviewed in connection
with devitrification we seem warranted in making the following
inductions :

(1) The vitreous state of a body represents a more

primitive and simple molecular structure with a
corresponding low degree of stability.

(2) There is such a thing as latent heat of vitrification,*

the loss of this heat being accompanied by the
building up of more highly-complex and more
stable molecules with a tendency to assume a
crystalline form.

(3) In some cases (as exemplified by Si (OH 4 ), H P O 3

and borax) hydration or dehydration as the
case may be appears to be a factor in the process
of devitrification.

From a somewhat different line of observations I have been led to see the
necessity of recognising two distinct modes of devitrification, as Prof. Bonney
has suggested (Address, 1885, p. 67). May we not distinguish these as
a. hydato-devitrification,
ft. thermo-devitrification,
and the converse in each case ?

Hydato-devitrification again may, it appears, occur in two ways :
(1.) by taking up H 2 O, hydro-devitrification:
(2.) by loss of H 2 0, dehydro- devitrification.

It is not unlikely that the non-recognition of (a) may have misled observers
in some instances, by suggesting changes requiring very high temperatures,
where no such conditions have really supervened in the history of the rock in
question.

In deep-seated rocks there is really no reason why, below the 'water-line of
permanent saturation' (Prestwich, Geology, Vol. i., pp. 164, 336), silica should
not remain for a great length of time as a colloid or imperfectly rigid glass ;f
in fact it is difficult to see how in some cases dehydration could happen at all.
This would be favourable to metataxic change without discontinuity of the
individual masses of the silica in such rocks.

An extension of the idea of allotropy to mineral chemical compounds and
the recognition of their susceptibility to metatropic change, along with the
general lowering of their fusion-temperatures as the result of the latent heat
contained in them in the vitreous state, is seen to throw considerable light upon
some of the obscure phenomena connected with the mechanical deformation of
composite rock-masses without crushing.

If we regard a glass as (per se) a (mechanically) stable liquid, that is one in
which the latent heat of liquidity is so far withdrawn that the molecules have
acquired such fixed relative positions that these cannot be altered without
fracture, may not movement of individual atoms still be possible, so as to build
up more stable configurations within the individual molecules, mtra-molecular

* I would suggest here (and especially in connection with the case of
cited) that this may throw some light upon the luminosity which calcite is
said to exhibit after cooling, below a state of fusion at a glowing heat in contact
with metallic iron ; also upon the recalescence or after-glow of iron itself in
cooling from a bright red heat.

t This idea suggested itself to me in the study of Prof. Bonney's description of the struc-
tural peculiarities of the Tor Cross Eock. (Q.J.G.S., vol. xl., p. 1&.)

42 ROCR-METAMOBPHISM.

as distinguished from inter-molecular energy still operating? Dr. Percy in
the volume 'On Fuel, &c.' (p. 54) of his 'Metallurgy,' states on the authority
of Fournet that 'the same silicates are more fusible in the vitreous than
in the crystalline state,' also that "when devitrified glass is heated it does
not soften before melting, but passes suddenly into the liquid state." He has
also observed acetic acid crystals retaining their form permanently at
temperatures at which the same acid when liquefied would not again crystallize.
Now since we have no reason for supposing that there is any difference in
the amount of heat latent in a given mass of a liquefied silicate, whether
liquefied by the fusion of the vitreous or crystalline form, it follows that
the excess of the quantity of heat applied to liquefy a given mass of a given
silicate (or mixture of silicates) when crystalline over that required to liquefy
the same mass when in the vitreous state must represent the latent heat of
vitrification. This seems to furnish direct proof of the existence of such latent
heat, and to suggest a method for its measurement.

In the light of the theory put forward in my paper on 'Dissociation and
Contact- Action ' (Chemical News,* vol. liv, No. 1402), as well as from
considerations already put forward in this note, it seems reasonable to regard
this residual latent heat as intra-molecular (atomic) kinetic energy. On the
principle of 'conservation of energy' it is represented by the equivalent of
work done in keeping down the atoms of the individual molecules in a lower
(less stable) state of combination, giving freer play to atomic forces.

A consideration of the known properties of artificial glass
throws some additional light upon the subject.

(a) In the dry way. The tension or strain to which unan-
nealed glass owes its brittleness is illustrated in a remarkable
degree by the disruption of a ' Eupert's Drop,' when the
point is broken off, from' the extremely unstable molecular
condition in which the surface particles have solidified. The
devitrification of glass and its conversion into ' Eeaumur's
porcelain,' by heating the mass strongly for a considerable
time embedded in sand or gypsum, was known a century and a
half ago. There have been various speculations (by Dumas
and others) as to what the precise nature of this change may
be, but they have now little more than a historical interest.
Among later workers Benrath has found that glass which
contains more silica than is represented by the formula M 2 Si 3 O 7
readily becomes devitrified. Leydolt maintains the exis-
tence of obscure crystalsf in ordinary glass, which have formed
in a super-saturated non-aqueous solution, since a crystalline
texture can be detected by the microscope on the surface of all
melted unpolished glass after contact with strong hydrofluoric
acid, and washing with weaker mineral acids. Peligot states
that the melting point of devitrified glass is higher than that
of the vitrified portion ; a fact which again suggests the idea
of latent heat of vitrification.

* Corrigenda in that paper :

p. 179, col. 2, line 21 from bottom, for 'variations' read 'vibrations.'

p. 180, col. 2, line 29 from bottom, for 'solubility' read 'stability.'

t Probably mere crystallites or belonites as in the case of sulphur described
above. It is very probable that these forms are developed (as Prof. Bonney
has suggested to me) by the action of the strong acid upon the glass, since
this is a mixture of silicates.

METATKOPY. 43

(b) In the wet way. Ordinary soda-lime or potash-lead-glass
is not only attached by acids, but is easily decomposed, when
placed for some time in contact with boiling water. Stas has
found that glass containing lead or alumina (the latter
found in almost all vitrified rocks) is readily acted upon by
acids, while Bohemian glass being rich in silica and free from
alumina withstands this action best. Here we have two
agencies : (a) heated water ; (b) acids capable of decomposing
a vitreous mineral. It requires no stretch of the imagination
to see how both of these may be present in the interstices of
rocks in the earth's crust to aid in the work of devitrification ;
nor must we forget the enormously increased value of the
factor of time to be allowed for in lithological changes.

The devitrification at the surface of glass which has been
buried for years in the surface-soil would seem to point to the
action of the humus-acids (which Julien* has shown to be in
the presence of ammonia great natural solvents of silica) as
playing a very important part in devitrification at no great
depths in the earth.

The action of superheated water in causing devitrification
doubtless by the enormous increase of its solvent power has
been fully proved by the splendid experimental work of M.
Daubree. By heating glass in water in powerful tubes and
maintaining them at a temperature of 400C with an internal
pressure of over 1,000 atmospheres for several weeks he
succeeded in completely decomposing the glass, the decom-
position-products consisting of

(1) alkaline silicates which remained in solution ;

(2) an amorphous opaque mass with a slightly fibrous

texture but otherwise resembling kaolin ;

(3) crystalline quartz, which so closely resembles

native quartz in crystal-form, cleavage, optical
properties, and even in the inclusion of globules
of water, as to be undistinguishable therefrom.
(Etudes experiment ales, pp. 159-166.)

Daubree further found that " the vitreous volcanic rocks
(obsidians and perlites), when submitted to the action of super-
heated water, appeared to behave in a manner comparable to
that of the artificial glasses " (p. 175). Assuming, as we may,
in cases of devitrification of the glasses of the older eruptive
rocks a considerable enlargement of both the factors of time
and temperature (the water possibly in some instances attaining
a red heat),t the experiments of M. Daubree would seem to

* " The Geological Action of the Humus Acids." (A. A. Julien) Proc. Am.
Ass. Science, 1879.

t See Pfaflf : Geol. als. ex. Wissenchaft, pp. 139-141.

44 ROCK-METAMORPHISM.

throw great light upon devitrification at depths, resulting in
the formation of perhaps some felsites and porphyries.

Bonney (Presidential Address, 1885, p. 66) mentions "a fair quantity of
earthy dust" along with globulites and belonites in a piece of glass which had
been heated for three weeks in a crucible to a bright red heat. This seems to
be a parallel case with the production of amorphous sulphur described above
(page 37.) Prolonged heating seems in both cases to have produced intra-
molecular change by bringing the atoms into new relationships with one another ;
whereas in an ordinary case of fusion followed by rapid cooling the changes
would appear to be rather of an inter-molecular nature. Time is thus seen to
be as important a factor in the thermal dj'namics of rocks as in ordinary
dynamical phenomena. It is the conversion of the absorbed luminous solar
rays into atomic kinetic energy which probably explains the formation of
amorphous sulphur on the surface of a molten mass congealing in strong
sunlight. The developement of very minute acicular crystals in the sheets of
window glass, described on p. 65 of Bonney's Address, was probably due to the
action of traces of atmospheric moisture condensed on the surfaces of the
plates of glass. This probably had something to do also with the
developement of the same structural character in the case of the piece of glass
described on p. 66, though it is not easy to suggest how this could have taken
place without knowing all the details from the beginning to the end of the
experiment. The comparison of those cases with that figured by Daubree
(Etudes Experimentales, p. 171) is interesting.

M. Daubrde says of the amorphous mass ('le residue fixe*') mentioned (2)
in the reference to his experimental work, that it is found on analysis to be
chiefly a mixture of hydrous silicates (p. 158), somewhat porous (p. 159), and
that a comparison of the analyses of the ordinary glass and the decomposed
glass shows that "le verre a perdu environ moitid de la silice et un tiers
d'alcali, et que le nouveau silicate a fixe' de 1'eau," (p. 162). He also points
out the zeoHtic character of the hydrated silicate. On p. 179 he makes the
following significant remark :

"En voyant le quartz se se'parer si facilement du verre, il est impossible de
ne pas reporter sa pense"e sur les veines de quartz qui sillonnent les quartzites
et les phyllades, et qui sont probablement formers, comme dans 1'expe'rience,
aux de"pens des roches avoisinantes."

How near he came to the recognition of the potent action of water in the
'critical state,' is seen from the following passage (pp. 169, 71) :

"Dans les experiences dont il s'agit, les deux tubes n'etant pas comple'te-
ment remplis d'eau, le tube de verre ne peut plonger dans le liquide que par sa
partie infe'rieure, aussi bien 1'inte'rieur qu' 1'ext^rieur. Cependant, il est
toujours attaque avec uniformit^ dans toute son etendue. Ce resultat prouve
que, dans les conditions ou nous avons opere, la vapeur d'eau, par suite de la
temperature, et de la densite qu' elle acquiert, agit chimiquement comme 1'eau
liquide. On entre alors dans un etat de choses ou la voie humide vient
presque se confondre avec la voie seche."

Dr. Percy in the first volume of his Metallurgy* has given
some very interesting information about glasses and slags,
which ought to be known to every petrologist ; and I have
seen specimens of devitrified glasses of extraordinary interest
and beauty in his splendid collection. Analyses of both the
original glass and of the devitrified (crystallized) nodules are

* London, John Murray, (new edition), 1875. See also his remarks (p. 54)
on the " Conditions which seem to be essential for devitrification."

METATBOPY. 45

quoted by him. These were made independently by Kersten
and Terreil. The latter analyst, by comparison of the results
of the analysis of the glass with the analysis of the materials
from which it was made, found that in the process of crystal-
lization there was no loss by volatilization. Dr. Percy points
out (p. 53) that the " crystallized glass may be regarded as
augite in which a portion of the magnesia is replaced by soda."
A comparison of the two analyses shows that the crystallized
portions are richer in lime and magnesia, and poorer in silica
and alumina, than the vitreous portions. This may perhaps be
accounted for by the unequal mixture of the ingredients in the
molten state.

It has been suggested by some writers that glass, being a
mixture of silicates, may be regarded as a solution of one
silicate in another. If so this may throw some light upon the
devitrification of natural glasses. But that it is not a necessary
condition of devitrification generally is clearly proved by the
facts reviewed in this work in connection with devitrification
of some elementary bodies (sulphur, phosphorus) and of silica
and arsenic. (See Appendix i.) Zirkel* refers to an ingenious
device of Vogelsang for illustrating the idea just referred to.
Vogelsang made separate solutions of sulphur and Canada-
balsam in carbon bisulphide ; he then mixed the two and
allowed a drop of the complex solution to concentrate on a
glass slide by evaporation of the carbon bisulphide. In this
way he obtained rhombohedric crystals of sulphur in a glassy
matrix. Greater interest however attaches (in the author's
mind) to the cases described in this work of the crystallization
of sulphur out of a matrix of the sulphur-glass itself.-^

In general (as Bammelsberg in his Miner alchemie points out,
p. 39) a body in the ' amorphous ' (vitreous) state is more
easily attacked by reagents than in the crystalline state. As
examples, he cites garnet, vesuvian, epidote, axinite ; all of
which furnish on melting glasses, which are decomposed by
acids with the separation out of gelatinous silica. To these
may be added the remarkable fact announced by Crookes and
Tidy at the Birmingham Meeting of the British Association
(1886), that powdered chalcedony was found to form readily
silicate of lead and so to purify water in which salts of that
metal were held in solution in minute quantities, while
crystalline quartz sand had no such effect.

Again, in such a mixture of crystalline and vitreous
(' colloid ') silica as is presented to us in common flint (as it
occurs in the chalk), something of the same sort of thing may
be observed. Long observation of the degraded flints of the

* Die Micros. Besch. der Mineralien und Cfesteine, p. 95.
"t* See Appendix i,

46 EOCK-METAMORPHISM.

gravels of the Bagshot country has made one familiar with a
great variety of appearances and mineral characters which
flints are capable of assuming, in what is usually called
' weathering.' This includes the long-continued action of
peaty waters or of humus-acids contained in the soil, which
with ammonia form soluble azo-silico-compounds. In this
way the successive layers of which the flint was built up very
often around a central mass a sponge it may be or an
echinoderm are brought out in a very marked manner by the
corrosive action of solvents.* In other cases the flints appear
to be so completely deprived of their vitreous constituents,
that they may fairly be spoken of as quartzite. It is not
impossible that some erroneous inferences have been drawn as
to the origin of some of the gravels by observers not sufficiently
familiar with these peculiarities. All degrees of degradation
of flint may be seen in them down to masses which are so
devitrified that they may even be mistaken for Sarsen-stones
of the more compact variety. On the other hand it would be
difficult to deny that the devitrified parts of flint may owe their
mineral character in part to direct metatropic change, by the
developement of a crystalline structure in the vitreous con-
stituents of the original flint. I have specimens of flint so
devitrified on both sides of cracks as to have acquired a stony
character through zones several millimetres thick. (See
further App. i., e.)

Turning again to the facts mentioned in this section of the
present work as to the relative densities of the different
allotropic forms of the same mineral, we are able to draw
from the fact that the maximum density and stability of
molecular structure is identified with the crystalline form the
deduction that pressure is favourable to crystallization. This
principle has received experimental verification in the cases of
quartz (Daubree), and perhaps of carbon in Mr. Hannay's
experiments in the year 1880 (Note L). And in general it may
be said that pressure tends in all cases to make any body pass
from a less to a more dense condition, as is well exemplified
(cf. also laboratory- work recorded in App. i.) in the case of the
liquefaction of ice by pressure a point which I have more
fully discussed in its nature and consequences in my paper on
the Mechanics of Glaciers.} If however we consider for a
moment the kind of pressure needed it may save us from some
false inferences. The pressure which liquefies ice is hydro-
static pressure : that is to say, it must act upon the mass

* A fine example of this structure is to be seen in the Museum of Geology,
in Jermyn Street; and corroded fragments of such 'banded flint' are by no
means uncommon in the gravels of Berks and Surrey. See Roth (Allgm. u
Ch. GeoL Vol. I. pp. 94-97) on ' Weathering of Quartz and Silica.'

t Quarterly Journal of the Geological Society, February, 1883.

METATROPY. 47

equally in all directions. It must in fact act by way of
compression, as in the experiments of Sir W. Thomson, Helm-
holz, Tyndall, and others. The ice must be confined in the
cylinder of the hydraulic press, or in some other way. Of
course in the early stages of the process we have fracture of
the mass, liquefaction at points of contact where the pressure
is exerted, and regelation of the liquefied portion as it escapes
into the free interstices (as in the well-known copper-wire
experiment) ; and in this way some loss of bulk is experienced.
But when the possible limits of this are reached in a closed
space, a further exertion of pressure can be made to liquefy
the whole mass, provided there is no escape for the liquefied
parts.

Just so, in the case of the mineral constituents of a rock
composed wholly or in part of vitreous or amorphous material.
We have no right to reason from the facts which furnish our
data to the inference that mere pressure can promote crystalli-
zation, unless that pressure be exerted upon the mass in all
directions ; in fact, by way of compression. We have no
warrant in assuming that a pressure lohich crushes a rock will
induce crystallization. It may act as an important antecedent
condition by allowing freer access and circulation of water
holding mineral salts in solution, and so prepare the way for
paramorphic changes; but a pressure so exerted cannot be
held to induce metatropic change. When these things are
considered, it will be seen that mere deformation of rocks by
pressure may have too much attributed to it, and probably has
recently had too much attributed to it as a factor in meta-
morphism. So far as the heat developed by pressure is
concerned, this is clearly adverse to crystallization ; its direct
effect tends in the opposite direction, rather towards fusion
than crystallization. On purely physical grounds therefore
we may demur to the notion that crystallization is caused
directly by deformation of rocks by pressure ; although it
would be rash to dogmatize on this question until the advance
of experimental physics has taught us something definite as to
the ' critical state ' of the passage of a body from a solid to a
liquid, and the reverse. Acting in the dry way it would give
us a dust, mere mineral matter in a state of molar division :
acting in the presence of water, it would certainly help to
prepare conditions favourable to paramorphic changes, because
it would give us locally conditions (heat, water, pressure)
approximating more or less to those which must have
prevailed universally at the surface of the globe when the
earliest crystalline rocks were formed.

That a mechanically-stable body like glass may not be at the same time
perfectly rigid is shown by the well-known fact that mercurial thermometers,

48 KOCK-METAMORPHISM.

if graduated soon after the tubes are filled with mercury, are liable to give after
a time too high a reading (to the extent in some instances of from 1 to 2),
from the compression of the bulb by atmospheric pressure. This brings out
very well the importance of the factor of time in vitrification, since it shows
that even after a vitreous body has become so mechanically stable as to be highly
fragile, a certain change (within very small limits) is still possible, under
prolonged strain, for the relative positions of its molecules. Is it not possible
that within such limits such forms as belonites may be developed ? [A series of
observations on thermometers which, having been graduated at once after being
filled, some of which were exposed subsequently (others not) to prolonged
temperatures at OC or lower, might give some interesting results as bearing
upon the theory of devitrification.]

As regards the necessity insisted upon in this work for hydrostatic compres-
sion as distinct from mere pressure, in promoting metatropic change qual
crystallization, the idea seems to have been even more strongly put by Prof.
Heim of Zurich. From a recent perusal of his great work ' Untersuchungen
iiber den Mechanismus der Gebirgsbildung ' I find that that distinguished
geologist emphasises, even more strongly than I have done, the necessity for
hydrostatic pressure. He shows how by this means deeply-seated rocks may
have been subjected for lengthened periods to a pressure far beyond the limits
of their rigidity (tiberlastet) ; and that this has given them a latent plasticity
which made mechanical (metataxic), and even molecular (metatropic), changes
easy in the subsequent massive movements concerned in mountain-building.
The possible ' critical state.' *It is not at all unlikely that certain minerals
contained in composite crystalline rocks may in some cases have reached this
state by the combined action of heat (tending to expand them) with great
(hydrostatic) pressure preventing free expansion. This is strongly suggested
(e.g. ) in such flattening-out of masses of quartz as is shown in the Atlas (Taf el
vii, figs 5, 6) which accompanies Lehmann's magnificent work Altkryst.
Schiefergest, as well as by the intrusion by pressure of adjacent mineral
particles into them. [The quartz of the larger pebble in fig. 5 shows no such
metataxic change, but only signs of fracture under pressure.] It seems almost
certain therefore that the flattened pieces of quartz have undergone the
particular kind of deformation which they exhibit from their having passed
through the 'critical state' (i.e. the state in which they were neither solid nor
liquid) owing to their having previously existed in a different allotropic condition
(hyaline or otherwise), which rendered the critical state possible for them, while
the temperature was not high enough for the same state to be induced in the
neighbouring quartz -masses, in which only crushing effects are seen. A similar
explanation may possibly apply to the deformation by stretching represented in
Tafel viii of the same work. It is not paramorphic but metataxic change
which is there observed, for the minerals (e.g. the garnets) were evidently
there previously completely individualized.t The stretching-out in continuous
bands of some of the minerals seems to show metataxis without disruption of
molecular continuity, according to the slight variations of the mineral-composi-
tion of the bands ; and this too may possibly be explained by those in which
(under metataxis) the continuity of structure has been most completely
preserved, having passed through the critical state. All traces of previous
allotropic variation in the quartz -fragments (Tafel vii) may have been disguised
by subsequent metatropy ;: but it is possible to understand how in some

*I had DO idea, when this was written, that the subject had been even touched by any
experimental investigator. It is therefore very satisfactory to learn that Amagat has shown
that carbon dichloride is solidified at a pressure of 900 atmospheres at 10C, and benzene at
22 C. under 700 atmospheres. He points out that there is "a temperature above which
solidification cannot be effected by any pressure ; that is to say, a critical point of solidification."
Comptes Rendus, July, 1887, quoted by Sterry Hunt, Chemical News, October 19th, 1888.

t I draw particular attention to the crushing, and even pulverization, of the garnets in
the non-quartzose layers, and to the preservation of their forms intact in the quartzose
layers, as seen (e.g.) in the specimen of 'normal Granulit' in the Lehmann Collection in
Jermyn Street Museum.

t In a note ou these, Lehmann says they have 'eine felsitische Beschaffenheit,

METATROPY. 49

cases the presence of traces of CaO or MgO, as in Asmanite, Faserquaru,
Chrysoprase (see Rammelsberg, Mineralchemie, pp. 163-4) may have increased
the tendency to fuse; while the presence of H^O, as in hyalite (ibid. p. 166,)
which loses 3-4 per cent on calcining, would seem from my experiments on
hydrated glass of Si(>2 (see Appendix i) to facilitate fusion. H. Rose too found
hyalite fusible to a porous glass in a porcelain furnace. *

In the light of these three principles we can, I think, explain satisfactorily
all the anomalies of structure which have been so carefully worked out by
Mr. Teallf (e.g.) in the case of the Scourie dyke, as well as those which have
been more recently described with great power by the officers of the Geological
Survey of Scotland.!

Thanks to the labours and the keen-sightedness of Allport as an observer,
we require no exercise of the imagination whatever to premise the existence,
in different parts of the same dyke (even as seen in the same quarry), or other
intrusive doleritic masses which have not undergone any considerable de-
formation by pressure, of such differences in composition and texture (vitreosity
principally) as our theory requires. The variable modifications observed in the
oldest eruptive rocks (basic and highly siliceous alike) afford no sufficient basis
therefore for any theory of 'regional pressure-metamorphism.' The one
stubborn fact in the case of the Scourie dyke which cannot be explained away,
and therefore presents an insuperable difficulty to the hypothesis that a
paramorphic change of augite into hornblende has followed as a result of the
pressure-deformation of parts of the rock, is the occurrence of the same mineral
composition in some undeformed portions of the rock as in those which have
been rendered ' schistose.' We may not perhaps find it easy, with ordinary
laboratory-appliances, to demonstrate the passage of mineral substances
through the 'critical state'; but we have certain and demonstrable data for
regarding it as in the highest degree probable. There is no unscientific use of
the imagination here : the only data we require are (i) that quartz and most
other minerals are fusible ; (ii) that their fusing-points are lowered by (a)
the presence of slight admixtures of fluxing-materials, (6) the previous (more
or less) vitreous condition ; (iii) that when fused at ordinary pressure they
expand. These data we have ; and the belief in the solid-liquid critical state
is as much an inferential truth of science, as our belief (by way of inference
from the results previously obtained by Faraday, Andrews, and others) in the

* Facts of a similiar nature to those just cited from Lehmann were
described and interpreted with a far-reaching prescience twenty years ago in
connection with the Huronian Series of Lake Superior, by Thomas Macfarlane,
Esq., now chief Analyst to the Dominion Government at Ottawa. Under the
name of Slate Conglomerate (the ' Slate ' being a sheared ' greenstone ' ) he has
described a schistose rock, in which, wherever the shearing has produced
marked ' schistosity ' the included granitic fragments are not only rounded off
into boulders by the rubbing down of their angles while the igneous matrix
was in motion, but in many cases have undergone such an amount of softening
that they have been drawn out into psbble-like masses, and even into flat and
twisted bands, a parallelism between these and the planes of schistosity being
always maintained. In some cases bands of what that author terms ' siliceous
slate ' are formed by the incorporation by fusion of the siliceous fragments and
[subsequent] shearing action ; but where the deformation of the fragments
without fracture' has taken place, I believe that the hypothesis of the solid-
liquid critical state furnishes the true explanation. See Mr. Macfarlane's
paper on the 'Geological Formations of Lake Superior' in the Canadian
Naturalist for May, 1867. I have been astonished, on searching in vain for
his name in the ' Index of Authors ' in either Green's, Geikie's, or Prestwich's
Geology.

t Q.J.G.S. vol. xli. pp. 133 et seq.
J Ibid, vol. xliv. pp. 391395.

50 BOCK-METAMOKPHISM.

liquefiability of the six reputed ' incoercible ' gases, was an inferential truth of
science, prior to the publication of the magnificent work of Pictet and
Cailletet in December, 1877.

By the recognition of these important physical principles :
(i) the latent heat of vitrification ;
(ii) the action of traces of fluxing-materials ;
(iii) the lowering of the temperature required for softening by incipient

fusion (following as a consequence of (i) and (ii) ) ;

new light is thrown over many obscure phenomena exhibited by crystalline
rock-masses, which have evidently suffered some deformation through dynamical
agencies.

Baltzer's observation that in mountain -movements limestones under great
pressure appear to have been the more flexible without fracture the richer they
are in argillaceous material (see Kalkowsky, Lithologie, p. 284), coupled with
the fact that in contact-metamorphism a calcareo-argillaceous cement of a
sandstone is often found melted into a glass (infra, vi.), seems to suggest
the passage through the critical state (under pressure) of the calcareo-
argillaceous material of such rocks, as the possible explanation of the facts
observed by Baltzer. This view is further borne out by some facts given by
Dr. Percy in his volume on Fuel, &c. (p. 75). Take for example the statement
that "any clay whatsoever may by the addition of from half to three-quarters
of its own weight of carbonate of lime, be rendered sufficiently fusible to allow
shots of metal to sink through the mass and collect into a button at the bottom."
This state of things would more than meet the requirements of the problem
presented by Baltzer's facts ; especially when we take into account the fluxing-
action (Percy, loc. cit.) of 'small proportions of different bases (e.g. MgO)
which are so often present in limestones.'

The idea of the possible passage through the critical state of certain of the
constituent minerals of the rocks is worthy then, I venture to think, of serious
consideration, in cases where great mechanical deformation (metataxis) is to be
observed, as in crumpling and puckering produced in the great earth-movements
concerned in mountain-building, but where at the same time the microscope
reveals no traces of molecular discontinuity (crushing) in the mineral bands.
In a heterogeneous rock it seems almost certain that this action would be
specialized, the induced conditions in a given proportion (heat, water, pressure)
causing some of the minerals to pass through this stage and not others. Nor
does the fact that the same minerals now exhibit a crystalline texture of
necessity militate against this view; since, with a subsequent lowering of
temperature owing to the slow dissipation of thermal energy after the pressure-
movement had ceased, we should expect the minerals to crystallize. A series
of chemical analyses of such portions of deformed rocks as are here referred to
(e.g. for traces of water of hydration or of CaO or other bases as fluxes in the
quartzose folia thus deformed) might lead to some highly interesting results.*
It is very likely that the 'mosaic-like' arrangement of the quartz-particles
frequently observed is only an incipient stage of contraction after the 'critical
state' causing shrinkage -cracks on a minute scale. It is almost certain that in
the first stage of devitrification such shrinkage -cracks do occur in some
instances (cf. Allport, Q.J.G.S. vol xxxiii, PI. xx, figs. 1-6).

Assuming that vitrification in the dry way is due to rapid cooling, a little
reflection will show us that this admits of degrees. If we take t^ =the initial
temperature of a fused mass when cooling commences, and ^2 the temperature
reached on cooling, we see that for a mass of the same mineral composition
and under the same pressure the value of ti must be pretty constant in every
instance, while the value of t 2 will vary considerably in different instances, and
will be determined (according to the general laws of dissipation of thermal
energy) by the resulting temperature of the adjacent cool bodies into which
the heat of the molten mass continues to pass until equalization of temperature

Leydolt's researches on Agates (quoted by Zirkel, Micr. BescU. der Min. u. Gest, p. 37)
seem to give support to this view.

METATROPY. 51

is reached. Again, the rapidity of cooling will depend upon the resultant of
the three factors; (1) the mass-ratio of the hot and cold bodies; (2) the
co-efficient of conductivity and capacity for heat (the two together giving as
the 'diffusivity') of the cold body; (3) the difference of temperature for the
time being. In cases where from any or all of these causes the cooling was
not so rapid as in others (as in the interior of such considerable masses as have
given us the pitchstones and obsidians), the early stages of cooling may have
been slow enough to admit of some incipient concretionary arrangement of
some of the constituents of the molten mass, sufficient to enable these
constituents to acquire such a degree of differentiation as would cause them
(with suitable fusion-temperature relatively to that of the residual magma) to
undergo incipient crystallization as the temperature fell. Such conditions would
probably give us the 'spherulitic structure.' On the other hand, in cases where
the value of t 1% was comparatively large and the cooling very rapid
(conditions of more complete chemical saturation of the base by the acids from
the accident of their existing more nearly in equivalent proportions in the mass
being also unfavourable to segregation) we should have a clear glass, which on
subsequent further cooling between the temperature of vitrification and 2 (in
such cases presumably considerably lower), would by shrinkage, followed by
the action of aqueous solutions on the shrinkage -cracks, give us the 'perlitic
structure.'

A beautiful example of incipient devitrification of the glass, of which the
vessels in use in a chemical laboratory are commonly made, has recently come
under my observation. An ordinary foot- jar had been used for the preparation
of hydrogen persulphide in the usual way, by the action of strong HC1 on
calcium pentasulphide, the jar itself being placed in a cold water-bath. It was
left standing after the formation of the hydrogen persulphide for some weeks,
during which all the persulphide formed underwent decomposition into free
sulphur and hydrogen sulphide. The jar happened to be slightly tilted in the
water-bath, and so many of the minute oily particles of the hydrogen persulphide
appear to have lodged on the inner surface of the lower side of the jar. The
glass was evidently attacked by these or by the nascent sulphur furnished by
their decomposition (or possibly by the Ca set free from the sulphide of calcium);
the result being a fine net-work of cracks on the inner surface (often with
beautiful iridescent fracture) and quite different from the scratches made near
the bottom of the glass by the harder glass of the stirring-rod used. From
various points in the walls of many of these cracks fine growths of belonites
have set up, which can be very clearly discerned with a moderate power of the
microscope, in some instances grouped into little nests. We are furnished
here, it appears, with a clear case of atomic movement in a (mechanically)
rigid glass giving rise locally to change of molecular structure, favoured
perhaps by local relief from the general surface-tension of the glass, under
the influence of which they had been previously kept in positions of
equilibrium.

The same sort of atomic movement is illustrated in the formation of micro-
liths in the midst of clear glass of tachylite. I was fortunate enough to
procure a very fine specimen of this mineral some years ago from the basalt of
the Falkenlei. The microliths developed in this glass occur isolated and in
bundles and bunches, the section furnishing a most beautiful object in
polarized light, especially when the microliths are seen in the dark ground-
mass of the glass between crossed nicols.

I have a specimen of quartz cut for the microscope which, from its
illustrating more than one kind of devitrification, seems of sufficient interest
to merit description here. As seen between crossed nicols :

(1.) On one side of (apparently) a shrinkage-crack is chalcedony, which
assumes in immediate contiguity with the crack an exceedingly fine mosaic-
like structure, this structure being more pronounced on the face of the crack,
becoming finer and finer as it gradually shades off into the non-differentiated
glass of the chalcedony.
E2

52 BOCK-METAMORPHISM.

(2.) On the other side of the same crack the same mosaic-like structure is
seen, but this becomes coarser as we recede from the crack, and presents a
gradual transition through quite large plates of clear glass (with very irregular
outlines, the lines of fracture being often beautifully iridescent) into a
continuous plate of clear transparent glass containing numerous minute
belonites.

(3.) Continuous with (2) is a ribbon-like banded structure of agate-
chalcedony, in which the bands are for the most part devitrified by the
developetnent in them transversely of an 'acicular mineral,' which reminds
one of the case described by Bonney (Pres. Address, 1885, p. 66). This
structure, through its whole periphery, shades off into the mosaic-like
structure.

The specimen appears to illustrate at the same time two distinct modes of
devitrification :

(i.) An anhydrous glass of transparent quartz losing latent (atomic) heat,
contracting and assuming the mosaic-like structure (thermo-devitrification).

(ii.) A hydrous glass of translucent quartz developing a fibrous (crystal-
loid) structure by loss of water (dehydro-de vitrification).

When we recollect that the feebly translucent form of silica, chalcedony, is
said by Rammelsberg (Mineralchemie, p. 163) to lose in some cases as much as
2 per cent, [of Water] on calcining, the hypothesis that the developement of
the transverse pseudo-fibrous structure seen in the agate-chalcedony is a case
of dehydro-devitrification certainly seems strengthened.

In a paper On Underground Temperature ' read by Prof.
Prestwich before the Eoyal Society in 1885, some important
facts are cited as to the circulation of underground waters,
going to show that in massive rocks such as granite very little
water passes ; while it passes more freely through phyllolithic
rocks such as slate, and still more ' rapidly through cross-veins '
and through zones of fracture and dislocation. The latter
is well known. In this freer access of water along the
junction-planes between massive crystalline rocks and rocks of
sedimentary origin composed of clastic materials we come to
see a very potent factor of local metamorphism, secondary
paramorphism through the agency of water (especially if
facilitated by previous crushing) followed by a welding
together (with a certain amount of shearing) of the new
minerals, producing zones marked by a certain degree of
' schistosity ' (with a rather free use of the term), without
producing a really crystalline schist. Some years ago in
examining a new section in the railway-cutting at Shap
Summit I traced the gradual change by mere ' weathering '
from the central exposed mass of unaltered dolerite through
partially disintegrated rock into wacke and mere clay; and
there seems no reason to doubt that, if this region were
subjected to a sufficiently powerful lateral pressure, we should
have in this case rocks, that would in some quarters be called
schists, formed in the inner zone and a phyllolithic rock
(slate) in the outer zone, if such antecedent changes occurred at
sufficient depth for earth-movements to operate upon the zone
of rock-material. (See further Section vi.) Again, every

METATROPY. 53

field-observer is familiar with the fact that along junction-
planes the rocks are generally very much decomposed, and
so rotten that (except in the rare cases of fresh and artificially-
prepared sections, such as the one just described) it is not
often that the true junction is seen (' the place being
overgrown') on account of the earthy material formed by
the disintegration of the rock on both sides of it ; and it is
to say the least extremely unsafe to infer a transition from
the clastic into the crystalline rocks from the appearances on
either side of, and at some distance from, the true junction-
plane. It may not perhaps be too much to say that this
is a by-no-means uncommon fallacy running through much
that has been written in support of 'regional metamorphism.'
And then the further inference is boldly drawn, that the
archaean schists, &c., may be only instances on a grander
scale of such transitional developement.

There does not appear to be always sufficient recognition of
the simple axiom, that where we see combined results it by
no means follows that they are referrible to one cause, or have
even been brought about by the simultaneous action of several
causes. In the class of instances here considered we see that
the combined results of paramorphic and metataxic change
may have been produced by several different causes acting in
succession, and very often at long intervals of time; the
causes which led to paramorphic change having been in
operation long before the pressure which has induced the
metataxic change (whether cleavage or foliation) was exerted
upon the rock-masses affected. This is borne out too by
Dr. Sorby's observations upon the microscopic structure of
clay-slate, which led him to the conclusion that the individual
minerals formed as separation-products in some clay-slates
were anterior to the developement of the cleavage, and had
only partaken under the influence of pressure of that
re-arrangement which we should call metataxic. Zirkel also
came to the same conclusion (see iv.). At the same time it
cannot be denied from the considerations adduced in the
foregoing part of this section that though the paramorphic
changes by which the new minerals were actually formed
were antecedent to the production of cleavage by pressure,
this agency may in cases where it could act hydrostatically
have helped to produce in them such a metatropic change as
would consist of the conversion (or partial conversion) of them
from the vitreous or amorphous into the crystalline state.

The physical law that pressure generates heat (as a
concomitant of molecular movement) is not overlooked ;
and it might be urged that this heat might be sufficient to
promote formation of minerals of more complex structure

54 EOCK-METAMOBPHISM.

out of those of a simpler molecular structure, as in the
formation in the dry way of silicate of lime and other
minerals referred to in App. ii, note A ; or perhaps in the
formation of enstatite (the purest examples of which are
found in meteorites, according to Eammelsberg) in the eruptive
rocks. We must distinguish clearly between quantity and intensity
of heat (temperature). In great earth-movements resulting
from lateral pressure working slowly and gradually through
a great period of time, it may be questioned whether
whatever the quantity of heat developed may be it would
be sufficiently concentrated to give the temperature required
by the hypothesis. Exceptional local cases of course may
occur where the movement is sudden along thrust-planes,
but we must not construct a theory out of these in favour
of ' regional metamorphism.'*

If moreover we consider a few of the best-known instances of
paramorphic change, one or two considerations are seen to
militate seriously against the view. As examples we may
take the conversion of augite into chlorite or hornblende,
and the conversion of olivine, augite, hornblende, and some
garnets into serpentine ; also the formation of many micas
as alteration-products of older silicates (Rammelsberg) . Of
the formation of mica in this way, even macroscopically, I
believe I have myself observed instances in the slaggy ropy
lava-flows of the old volcano Falkenlei near Bertrich, and
among the disintegrated debris of augitic lavas at Daun.

In all the cases here cited of paramorphic change water
of constitution is taken up as one part of the process ; and
ordinary laboratory-experience is opposed to the notion that
this is aided by heat. But it might be urged that these
changes are presumed to take place under conditions as to
pressure of which such experience takes no account. Primd
facie this seems feasible ; but to this we may fairly object
that if such were the case the resultant minerals ought to
have a greater specific gravity than the originals, whereas
the facts of the case show the contrary. A consideration
of the following simple table will put this in a clearer light,
the percentage of water of constitution being given as the
mean of a number of analyses, in the case of each mineral,
as given by Eammelsberg ( Miner alchemie) :

No more can we by any process of sound inductive reasoning build up such a theory upon
any foundation in the facts observed in such folded anticlinals as occur (e.g.) in the Alps,
in the Urseren Thai between the St. Gotthard and Finsteraarmassif, and between the St.
Gotthard and Tessinamassifs, on either side of the Gotthard-axis of elevation. On a
' regional' scale the real problem to be solved is the genesis of those huge massifs themselves,
and of the characters they possess in common with the great archaean crystallines in all the
four quarters of the globe, and not the origin of such inconsiderable localized masses of
altered sedimentary materials as have been here and there squeezed as in a vice between them.
(See further App. ii, Note P.)

METATROPY.

Mineral.

Water of Constitution

Specific Gravity.

Garnet

q.n A.O

U per cent.

O Z 4 O

Olivine 1 -

3-4

Augite 2

33-5

Hornblende 3 - - -

( generally )
< less thanl >

( rarely, 2 )

Muscovite - -

4-08

2-9

Chlorite - - - -

12-11

2-8

Serpentine - - - -

13-08

2.6

1 Chondrodite (of close mineral affinities with olivine) contains no water.
Villarsite, which agrees with olivine in crystal form and optical characters,
contains 4 to 6*2 per cent, of water, with a corresponding diminution of
density to 3'04.

2 Occasionally a trace of water, rarely as much as 1 per cent.

3 Weathered varieties are found to contain 3 to 20 per cent.

The obvious general conclusion on this point would seem
to be, that though pressure acting hydrostatically is favourable
to crystallization it can only be safely asserted to promote
this by way of a metatropic alteration of bodies whose chemical
composition has been previously determined by various para-
morphic agencies ; that its action in this respect is limited to
those cases in which crystallization is accompanied by increase
of density of the body which crystallizes ; and that this stops
a long way short of what is required of it to bring about
those vast and complicated changes which are implied in the
' metamorphism ' of clastic sedimentary rocks into crystalline
schists and gneiss.

IV. METATAXIS.

This term has been already defined as connoting alterations
in the relative positions of the constituents of a rock-mass ;
and the developement of slaty cleavage, which is now pretty
generally recognized as a result of mechanical force, has been
instanced as typical of metataxic change.

a. Cleavage. The recent exhaustive paper on this subject
by Mr. Barker* has put the matter in a very clear light.
While agreeing with that author in his general treatment of
the subject, and in his general conclusions (so far as cleavage-
proper is concerned), I submit bhe following remarks:

* British Association Report (1885), Aberdeen Meeting.

56 EOCK-METAMOEPHISM.

Instead of contending for the absolute truth of one
particular form of the ellipsoid of strain, it will be seen that
in some instances (from variations in the determining factors)
Dr. Haughton's view might be true to the facts of nature,
as represented by an ellipsoid of rotation; i.e. with equal
axes parallel to the ' end ' and ' side ' ; while it would fail to
be true in cases where the movement of the constituent
particles was free only along the line of dip. Eesistance
being in the latter case offered by more rigid masses of the
crust of the earth on either side of the region or zone of
compression, the movement in the direction of the line of
strike of the cleavage might be sufficiently hindered to allow
of no bulging-out in that direction. This would (the operating
force being presumably the same in both cases) give a greater
elongation to the ellipsoid in the direction of the cleavage-dip.
On the other hand, if the particles were comparatively free to
move in the direction of the cleavage-strike, the portion of
the compressing force expended in moving them in that
direction would of course not be available for moving them
in the direction of the cleavage-dip. In this case, the
striation or ' grain ' of the slate would be less pronounced.

Again, the ratio which the extension of the rock-mass along
its line of cleavage-dip would bear to that of compression on
the one hand, and of extension in the direction of the cleavage-
strike on the other, must be in part determined by the relation
which the pressure might bear to the dead weight of the
superincumbent mass. It is conceivable that at great depths
lateral pressure might result in producing a cleavage-structure
in a plastic or pasty rock-mass, under such a dead weight of
the superincumbent mass that the movement of the con-
stituent particles of the rock in the direction of the cleavage-
dip would be so far retarded that we should not have a
strain-ellipsoid at all, but an oblate spheroid of stress.
Mathematical ways of looking at natural phenomena give a
certain fixedness and precision to our ideas ; but nature is
not bound by them ; and there is certainly a danger of the
mind which is too much entrammelled by them losing some of
that elasticity which it might otherwise apply to the study of
natural phenomena.

In dealing with purely geometrical considerations, as (e.g.)
in determining the centre of gravity of a solid homogeneous
body the mathematical conception of the body as made up of
indefinitely thin parallel planes is no doubt a safe one ; but in
considering the causes which have induced such metataxic
disposition of the constituents of a rock-mass as are exhibited
in slaty cleavage, we are dealing with dynamical phenomena
on a minute scale. The question arises, whether our conception

METATAXIS\^ Q J 57

of such shearing-planes as are postulated in Mr. Fisher's
theory, which attributes cleavage to the shearing-action arising
from the settling down of the mor.e central parts of an
elevated mass a sort of magnified ' cone-in-cone ' structure
is anything more than a mental fiction. A negative answer to
this question would seem to be furnished by the simple
consideration that such a strain as would be caused would be
continuous through the mass, and would result rather in a
slight (perhaps hardly perceptible) strain -structure, rather
than in that discontinuous mode of metataxis which we
recognize in slaty cleavage. And there is the further fact
militating against Mr. Fisher's theory, that it is generally
(though not universally) found that it is in the oldest
argillaceous rocks, such as the Cambrian and Silurian, that the
phenomenon of slaty cleavage is most distinct and perfect.
It seems to be sometimes forgotten that such rocks are now
found near the surface of the earth as the result of extensive
denudation ; they are but parts of the worn-down stumps of
ancient mountain-regions from which some thousands of feet
of superincumbent strata have been removed since the
cleavage was induced upon them. But if Mr. Fisher's theory
were the true explanation of the fact, we should expect surely
to find things exactly the reverse of this.

It has been pointed out above that the settling-down of the
more central parts of a mountain chain would tend to produce
rather a feeble strain- structure than cleavage. This implies
of course that the materials were still in such a plastic state
as to admit of such a metataxic change occurring. In other
cases the rock, from loss of interstitial water after its elevation
(as pointed out long ago by Von Cotta), might become too
brittle to allow such an alteration in the relative position of
its particles, or even bending and contortion ; and then we
should have an indefinite number of small fractures combined
with a certain amount of sliding and possibly slickensiding
along some of the cleavage-planes, which would give us (with
subsequent cementation) the phenomenon of a number of
minute step-faults. * This would furnish literally a case of
' Ausweichung,' and so the term Ausiveichungsclivage' would
be fit and appropriate. It is unlikely that any great mass of
rock would become equally rigid throughout at the same time ;
and so we should be prepared to find that in the settling-down
of the central parts (a fact it would be difficult to deny) the

* The mode of fracture and recementation here referred to is a matter of
common observation. I have seen it exhibited in the Cumberland slates and
even more extensively in the limestones of the Northern Alps, both in the
rock-mass and in boulders. The recementation usually occurs in the lime-
stones through the deposition of calcite. It is a different thing from a
recemented ' crush-breccia. '

58 KOCK-METAMOKPHISM.

parts which remained more plastic would undergo minute
contortions, such as those figured by Mr. Marker in his
invaluable memoir (Figs. 11 and 12) ; while the gritty bands
in the slate, possessing less cohesion, would furnish zones
of least resistance to the lateral strain, the angle of their
previously induced cleavage being changed without that
cleavage being of necessity obliterated. This would give us
the " zigzag arrangement " noticed by the same writer (Fig. 13).
While then there would seem to be insuperable difficulties in
the way of attributing cleavage-proper to the cause assigned
to it by Mr. Fisher, it would seem that there are other and
subsidiary phenomena which may be attributed to its operation ;
and if we can see in this way the work done by its operation
there remains the less necessity for seeing that work done in
producing the primary cleavage itself. In the present state of
our knowledge of this subject it seems to be pretty generally
acknowledged that pressure is a sufficient vera causa for the
phenomenon of cleavage ;* the fact that the cleavage has
been induced subsequently to the displacements of the rock-
masses by contortion and faulting simply implies the continued
operation of lateral pressure after those limits were reached,
within which displacement of the rock-masses in each case
was possible. No general rule can be laid down as to what
those limits would be. If the pressure ceased before those
limits were reached we should have (as is so common in the
Secondary Eocks) ordinary contortion or faulting (or both),
without cleavage ; but in such a case we clearly get no
cleavage because the force in operation is being expended in
doing the work of displacement, and consequently is not
available for the other kind of work at the same time.t

To put it another way, while the work of mere displace-
ment is going on, the action of the lateral thrust is greater
than the reaction due to the resistance of the mass ; but
when that mass has by crumpling and fracture (both on the
faulting and on the crushing scale) been so rammed together
that further displacement of rock-masses is impossible, the
reaction equals the action of the thrust, and then cleavage-
structure begins to be set up. The production of cleavage
after the displacement of the rocks en masse, which the

* See App. ii, Note M, on the distortion of fossils.

t It will be recollected that the same line of reasoning was adopted by the
writer to explain the distribution of, and the work done by, the mechanical
force of the moving mass, in the 'Mechanics of Glaciers' (Q.J.G.S. February,
1883). It is satisfactory to note that the main contentions of that paper are
borne out by the work of Prof. Penck, in the Northern Alps, and by the
thorough-going work of Prof. J, W. Spencer, in Norway, (see his ' Glacial
Erosion in Norway' ; Proc. Roy. Soc., Canada, 1887). The latter author has
however slightly mis-quoted me on the matter of solar radiation.

METATAXIS. 59

generally-observed relation of the cleavage-planes to the
contortions plainly indicates, is thus found susceptible of
explanation, and lends no support to Mr. Fisher's theory as to
the origin of the cleavage.

It is with some diffidence that I ventiire to say that Prof. Heim's
arguments (Meek, der Gtbirgsbildung, Bd. ii, p. 12) by which he supports the

proposition, "Alle Umformung es heute vor uiis stehen,"

seem to me not altogether convincing. Still less convincing do they become
when one works through the details of his illustrative figures (ATLAH,
Tafeln xiv, xv) in the full recognition of the vast physical difference there is
in the molecular structure of the same chemical body according as it is
crystalline or amorphous.* The difference in the molecular mobility (especially
with access of water) in the two cases is enormous. The expression "in
gleichem Grade fest und hart" requires this large qualification as well as that
implied in the qualifying word "annahernd."

The flattening-out of the particles and their general
re-arrangement with their longer axes in the direction of the
cleavage-dip can scarcely be said to receive much elucidation
from Tyndall's illustration of the action of the rolling-pin in
producing the laminated structure of puff-paste, since we are
not at liberty to postulate the operation of such a force as
' rolling-friction ' in producing slaty cleavage.

In Ramsay's Memoir on ' The Geology of North Wales '
some interesting instances are described of cleavage induced by
pressure upon the volcanic ash-beds of Snowdon. In these
cases no mention is made of the occurrence of accessory
minerals on the cleavage-planes, nor of anything approaching
to foliation. In the case of the ' porphyries ' which are stated
to be thus affected by cleavage in common with the altered
ash-beds there is some obscurity arising from the dubious
nature of many rocks which in the older nomenclature have
been described as porphyries ; but in the case of the cleavage
of the ash-beds there would seem to be little difficulty in
understanding its production, when we recollect the great
permeability of volcanic ashes to water. This has been
recently pointed out and important deductions drawn from it
as the main factor in determining the intermittent action of
volcanoes, by Prof. Prestwich in a paper read before the
Royal Society in 1885. Given sufficient time, pressure, and
temperature (as determined by depth) we have all the factors
needed for the paramorphic changes called into play in con-
verting a loose incoherent mass of volcanic ash into a hard
compact semi-crystalline rock, which gives these Snowdon
altered ashes great power of resistance to the hammer (as I

* My impression is that the figures certainly indicate a bulging inwards of the slaty
matrix of the rock between the displaced segments of the Belemnites indicating plasticity,
and this impression has been confirmed by an examination of the specimens which Dr.
Heim exhibited at the International Geological Congress in London, 1888.

EOCK-METAMOEPHISM.

know from personal experience) ; while the ash itself retains
distinct traces of its original stratification. Among the para-
morphic changes wrought by the action of heated water
would be the separation-out of silica, to form the cementing
material to which the rock owes its present indurated
character. If, while this process was going on, the mass
was subjected to great lateral pressure, the silica being
still to a great extent in the colloid state, it is easy to see
how the cleavage structure may have been caused ; and there
remains no necessity for inventing a theory to account for
the production of cleavage in a rock of the indurated
character which these ashes now possess. It is possible
that the pressure itself which produced the cleavage may
have helped the process of induration by the expressure of
the water of hydration of the colloid silica, thus effecting
a certain amount of nietatropic change as well as a metataxic
change in the rock mass. It would thus seem that cleavage
instead of crushing has resulted, because the rock was not
at the time sufficiently rigid for the latter result to occur.
In this case we look in vain for any paramorphic change
which we can attribute directly to the action of pressure.

There are two other considerations which militate strongly
against the view that the accessory crystalline minerals
found on the cleavage-planes of many slates are the result
of paramorphic changes induced by the pressure itself.

(1) These minerals have generally a definite orientation
in the direction of the cleavage dip, as if in the shearing
process which produced the cleavage they had moved into
the position of least resistance before the rock-mass became
too rigid to allow such movement.

(2) Similar minerals are found (as before noted) as
separation-products in laminated shales (Schieferthon) and
on the cleavage-planes of true slates (Thonschiefer). The
following are noted from the last edition of Credner's Geology
(p. 123) :

ACCESSORY MINERALS IN

THONSCHIEFER.
(Silurian and Devonian.)
Rutile (as prismatic needles parallel

to the cleavage).

Plates of green and yellowish mica.
Scales of felspar.
Oval and roundish grains of quartz

with numerous fluid inclusions.
Pyrites.

ACCESSORY MINERALS IN

SCHIEFERTHON.
(Secondary and Tertiary.}
Microliths of hornblende.
Scales of potash-mica.
Particles of quartz.
Minute laminae of iron-glance.
Brownish and greenish needles of

undetermined nature.
Pyrites.

Von Hauer (Die Geologie, p. 55) mentions the common
occurrence of such minerals both in Thonschiefer and
Schieferthon.

METATAXIS. 61

Zirkel (Die Microscopische Beschaffenheit der Mineralien und
Gesteine, Leipsig, 1873) describes the occurrence of such
micro-crystalline constituents of Thonschiefer. Of the alter-
native views as to whether these minerals previously existed
in the unaltered rock, (' vor seiner Verfestigung '), or were
formed on the other hand subsequently by metamorphic
processes ('metamorphische Vorgange '), he decidedly inclines
to the former view. His words are (p. 494) : " Jede sorgfal-
tige Untersuchung der Beschaffenheit der Diinnschliffe, jede
vorurtheilslose Betrachtung der Anzahl, Lagerungsweise,
und Vertheilung der krystallinischen Elemente, welche sich
schwerlich erst in dem starren Gesteine hinterher entwickelt
haben, hat bis jetzt mit der Ueberzeugung geendet, dass
der erste Theil jener Alternative ebenso warscheinlich, als der
letztere unwarscheinlich ist."

A distinction between the phyllites and Thonschiefer (slates) is thus drawn
by Kalkowsky ( Lithologie p. 256J : "While the phyllites (Urthonschiefer) are
throughout purely crystalline rocks, the Thonschiefer consist mostly of derived
(allothigenous) constituents as well as of authigenous, though there are cases
in which the former appear to be wanting." He also states that "a widely-
spread constituent of Thonschiefer is a colourless or faintly-coloured micaceous
mineral, the feebly-refractive scales of which usuall} 7 lie parallel to the cleavage-
planes. Such scales are probably in many cases derivative (allothigenous) ; the
chief part of the micaceous mineral appears however to be in many cases
authigenous."

Bonney (Pres. Address, 1886, p. 66) says that his observations fully "confirm
Sorby's statement concerning the presence in many cases of exceedingly
minute flakes of a micaceous mineral in large quantities;" and hints at their
possible derivation from the felspathic materials in the original clay. "Still
exogenous (allothigenous) mica may be often observed." He remarks that "the
crystallization of the mica is no doubt a consequence of pressure" (metatropic
change). The mineral as a chemical body was evidently there (in the case of
secondary or authigenous mica resulting from secondary paramorphic change)
before the cleavage of the rock- mass was induced by pressure.

f3. Crumpling and gnarling. Another variety of metataxis
is seen in the crumpled and gnarled condition of the folia of
many gneisses and schists. That the pressure which caused
this has not been the cause of those paramorphic changes by
which the minerals of the rock were formed is seen in the
simple fact that its action was (at least in a very large number
of instances) posterior to their formation. It must inevitably
occur that in the history of such rocks they have been
locally subjected to a ramming process, the pressure acting in
one direction upon them for a length of time, while the
reaction due to the resistance of bounding rigid masses was
exerted upon them in several directions. We may illustrate
this by the compression which a lump of clay suffers when
it is forced into a brickmaker's mould. If the clay is
homogeneous we get a homogeneous brick ; but with a

62 ROCK-METAMORPHISM.

laminated clay which has been imperfectly ground and
mixed the case is different. Of this I have lately observed
some instances, in the case of bricks which have been
undergoing weathering in the face of a wall for some twenty-
five years in this neighbourhood, and were made as I
have ascertained from the laminated clays of the Bagshot
formation. The surface of these bricks exhibits a contorted
and gnarled structure as complete and marked as is to be
seen in any schist or gneiss that I have ever observed.
The heat of the kiln has in this case rendered the contortions
due to compression permanent ; but it has imparted to the
mass nothing of the nature of foliation.

Interesting examples of crumpling, with parting-asunder of the folia at the
acuter portions of the curves and filling-in of the interspaces with injected
granite (Granit injicirt) are given by Lehmann (Atlas, Taf. ii, fig. 1 ; Taf. xiii,
fig. 2). On Taf. xvi, fig. 2, is shown a very interesting example of " Granulit
durch Stauchung zerissen, die Lucken mit secundaren Quartz erfullt." A
comparison of such beautiful examples of crumpling without fracture (bruchlose
Faltung) as are given on Tafel xvi, tig. 4, Tafel xiii, figs. 1, 2, 3, Tafel xix, figs.
2,5, with the examples of crumpling with fracture given on Tafel xvi, fig. 1,
Tafel xiv, figs. 1,2, is very instructive. A slight difference in the previous
allotropic condition of the minerals (chieHy the quartz) and the consequent
difference in the degrees to which they lent themselves readily to the critical
state, seems to have made all the difference in the two phases of change
illustrated in these two series.

The frequent flexure (and contortion) of massive quartz veins in the phyllites
and slates of the Alps, such as one often sees in the fine artificial sections
made in road-cuttings, without any apparent signs of fracture, really presents
no difficulty, when we allow for such slight meta tropic change in the quartz
itself as might easily result from the loss of water during the later process of
induration resulting from desiccation, which they have undergone in common
with the mountain -masses in which they occur, since their upheaval. Labora-
tory-experience (see App. i) bears out this view.

y. Foliation. Teall* has lately described an interesting
experimental illustration of the effect of pressure upon a
mixture of clays differently coloured, in inducing ' parallel
banding ' in such a mass. But a little reflection shows that
this does not carry us very far as a clue to the way in which
the foliated structure of the schists and gneisses was brought
about. For, (1) some distinction (if any) should be noted
between the effects of squeezing and rolling ; (2) we can
hardly (as before pointed out) invoke the action of rolling-
friction on any large scale to produce the shearing which
is probably the immediate cause of the lamination or parallel
banding observed ; (3) the application of pressure by rolling
is clearly intermittent, and so time is allowed between
successive applications of pressure in this fashion for the

* Geological Magazine, December, 1887.

METATAXIS. 63

movement of interstitial water and the action of capillary
forces to bring about a transfer of the colouring-matter ; (4)
the real crux of the problem the conditions which caused
the assumed antecedent pasty or yielding condition of the
rock-mass is left unexplained, and we must look for it in
another direction. Let us consider the fact that recent
extensions of our knowledge of the history of the Earth's
crust tend to show that the movements which have resulted
in the formation of the central cores and the great mountain
back-bones of our present continents, and of continental
tracts of which only fragmentary portions now remain as
' dry land,' were in most cases initiated at an early stage,
probably before the close of the Palaeozoic Period, to take
a wide limit. This not only follows as an inference from
the vast amount of accumulated material which has been
derived from them to form those subsequent sedimentary
deposits to which we cannot assign an organic or chemical,
but only a mechanical origin ; we have direct evidence of it
in the vast amount of rolled detritus found all over the world
among the palaeozoic formations. As a few examples of this
we might point to the great breccias and conglomerates which
mark the latest palaeozoic epoch (the Post-carboniferous) in
the Eothliegendes of central Germany, in the great zone of
such derived and rolled materials which marks the same
epoch in the Alps from end to end of the system; in the
great ' Permian ' conglomerates of the West of England and
(according to the view put forward by me elsewhere) of the
Devon area.* Everywhere the derivative relation of the
materials of these detrital rocks to the palaeozoic and archaean
rocks of the adjoining regions is the same. And, as is well
known, conglomerates and breccias occur more or less in all
the palaeozoic formations down even to the Cambrian, which
are quite distinguishable from the volcanic breccias of those
same formations, with which we are not here concerned.
Sub-atmospheric waste and degradation implies of necessity
previous elevation ; and so it would seem impossible to deny
that before the close of the Palaeozoic Period extensive
portions of the primordial archaean floor were worked up to
the surface. And if we are justified in making deductions
from the conclusions arrived at in Section ii. of this work,
there really does not seem to be any very great difficulty
in accounting for the non-rigid character required for the
metataxic changes wrought in those rock masses to produce
foliation, as they were squeezed up from beneath overlying
strata, in the initial movements which marked out many

* See Journal of the Geological Society, vol. xliv., ' On the Red Rocks of
the Devon Coast Section.'

64 EOCK-METAMOKPHISM.

of the great mountain-systems of the globe. When we
consider the heat that must have been as yet retained in
in those buried masses, owing to "the low coefficients of
conductivity of the materials which covered them, and the
great capacity for heat of water which must have necessarily
made the cooling down of oceanic waters a very slow process,
and add to this the fact of the long continued saturation
of those archsean masses by water so long as they remained
below the level of the sea, it can hardly be considered an
extravagant or unwarrantable assumption that those rocks
continued for the most part, prior to their protrusion in the
early formation of mountain-systems, in such a condition
as to meet all the requirements of those rnetataxic changes
which appear to have been produced by the mechanical
agency whereby their upheaval was effected before the close
of the Palaeozoic Period.

We must also be prepared to recognize the effect of
long-continued pressure upon such rock-materials, in accen-
tuating any incipient tendency to such a taxic order as may
have resulted from the circumstances under which these
materials were deposited, as the minerals produced by
paramorphic change among them assumed a more distinct
and definite individuality. This has been already touched
upon in Section ii. of this work ; and in Section iii. the effect
of (hydrostatic) pressure has been recognized as aiding crystal-
lization of nearly all mineral bodies except that of water
itself, the peculiar function of which in this respect (that
is to say, its liquefaction by pressure) is only one of the
many known properties of burnt hydrogen, which make it
the most wonderful (though the most common) and most
important chemical body in the whole economy of nature.
If such an incipient tendency existed in the materials of
the fundamental schists and gneisses, it can hardly be
doubted that long-continued pressure would tend to develope
such a structural character. We may parallel such a case
by what we know by direct observation of the difference
in the degrees of definition of the lamination of a Coal-
measure 'chinch,' a Liassic 'shale' and a Tertiary ' laminated
clay.'

These considerations are suggested as throwing light upon
the fact that the same materials are found in the non-foliated
granitic rocks and foliated gneisses, with the frequently
observed fan-like arrangement of the latter on a large scale,
in the central crystalline axis of such great mountain-ranges
as the Alps : and they meet, I venture to think, the case of
the rocks in the Cornish peninsula described by Mr. Teall
(op. cit.), since these are but parts of the core of a great

METATAXIS. 65

mountain system,* which was initiated (as we know from
physical evidence) in palaeozoic times.

8. Metataxic work done by Solar and Lunar Tides. On
thinking over the various mechanical and physical forces
which must have acted upon the earth in the early stages
of its developernent, it has occurred to me that geologists
have made too little of the new insight into the past which
has been gained by the recent researches of Prof. Charles
H. Darwin, as to the cycle of changes that have been
going on concomitantly in the length of day and night,
the moon's distance from the earth, and the time of its
revolution in its orbit. Taking his stand upon the results
obtained from such purely mathematical and physical data,
Prof. E. S. Ball five years ago gave us such a " Glimpse
through the Corridors of Time," as made the more extreme
doctrines of the Huttonian school of geologists totter on
their pedestals. A general laissez-faire sort of acceptance of
the views of the more advanced metamorphists consoled
some of us perhaps with the notion that it did not matter
much whether tides had been greater agents of degradation
of denudation and of transport in the remote ages of the
Palaeozoic Period than in more recent times, since it was
assumed that the oldest rocks we know were derived from
other still older rocks. As however the argument has
developed itself in this work, we have been led to see that on
many grounds there are grave reasons for rejecting such a
doctrine.

The idea of the work done by solar and lunar tides upon
the non-consolidated magma in the archaean and pre-archaean
stages of the Earth's evolution has arisen quite independently
in my own mind in the course of writing the thesis on which
this work is based. f

Taking the general idea of Prof. E. S. Ball as it was
published by him, and accepting the general principle, we
must demur to one of the results speculatively arrived at by
that distinguished astronomer. He applies a dynamical prin-
ciple quantitatively, and from the data furnished by a very
moderate tidal wave of the present ocean arrives at the

* The recent work of Prof. Bonney in South Devon has placed this in a very
clear light. See his paper on ' The Geology of the South Devon Coast '
(Q.J.G.S. vol. xl.) also the paper by the same author on 'The Older Rocks of
Brittany ' (ibid. August, 1887.)

t In the course of the passage of this work in its present form through the
press I have discovered that the idea of a mobile magma was thrown out
more than twenty years ago by Macfarlane, ' Origin of the Eruptive and
Primai-y Rocks,' pp. 62, 63. See also Naumann, ' Lehrbuch der Geognosie,'
quoted by that author, (loc. cit.)
F

66 BOCK-METAMOKPHISM.

conclusion that the tidal wave must have been so many
millions of years ago so many hundreds of feet high. It
would be perhaps impossible to challenge this deduction,
could we only assume that the quantity of water condensed on
the globe was the same as at present. This manifestly by his
own showing we have no right to do ; and so we must call
upon Prof. Ball to take this very large qualifying circumstance
into account. When however this is done, we can still leave
him a wide berth for the dynamic action of the oceanic tides
of the palaeozoic and later pre-Cambrian ages. But as
geologists, and especially in connection with our present
subject, we begin to suspect that Prof. Ball has only told us
half the story. He tells us of the enormous action of the
Moon upon terrestrial waters, when that orb revolved near
the Earth at such a proportionately greater velocity as is
required by Kepler's law of " equal spaces in equal times."
We go further back, and ask what was the effect of the
Moon's attraction both on the magma and on the dense
atmosphere which enveloped it, at a period still more remote
from the present ; at a time when the surface of the globe
was too hot for the condensation of oceanic waters, or for
any other water than such as was entangled mechanically
among, or combined chemically with, the mineral constituents
of the molten or semi-molten lithosphere ; for such he con-
ceives to have been the condition of things at the surface of
our planet for some time after the * Moon's birth.' And upon
the back of this comes another question, as to what was the
work done by the solar tides, of the existence of which he
tells us, in the non-consolidated crust, even at a time anterior
to that portentous event. May it not be suggested that, when
the answers to these questions come to be carefully thought
out, it may be seen that the action of those primeval tides
must have resulted in extensive metataxic change in the non-
consolidated materials of those fundamental rocks, and that in
this we have at last found a clue to the mystery of their
foliation ? In such an unequally viscous mass there would
be tension, contortion, and shearing to any extent during the
tidal pulsations which the magma was suffering. The more
yielding portions would participate more readily in the
undulations, and would therefore shear over other less yielding
portions with which they were in contact. Portions already
solidified, or nearly so, by segregation or otherwise, as time
went on, would by their vis inertia present obstacles around
which a fluxion structure would develope itself in the
contiguous portions of the yielding magma, giving us perhaps
in some cases ' Augengneiss.' The local tension of parts of
the viscous lithosphere especially near the crests of the waves,

METATAXIS. 67

would imply stretching and consequent lowering of tem-
perature, a circumstance favourable to local solidification.
Who shall say that in the later and feebler struggles of this
kind, as secular cooling went on and the magma approached
nearer and nearer to the conditions required for consolidation,
some of these tidal waves may not have become in situ
sufficiently rigid to outline some of the earliest lines of
elevation? *

The term ' Augengneiss ' has a definite use in German petrography, and is
properly used to denote those gneisses " in which single large felspar-masses
(orthoclase), in form ranging from the blunt lenticular to the approximately
spherical, occur, segregated from the matrix of the rock which possesses a
schistose (schieferig) or coarse grained (flaserig) structure, and around which
the mica-laminae adhere." (Credner, op. cit., 6th ed., p. 101.) Such a
structure could easily be developed in parts of a slowly-cooling siliceous
magma moving differentially under pressure (shearing) ; and the difficulty of
conceiving the same segregation-structure as developed by the deformation of
a solid rock-mass under pressure seems to leave us very good grounds, so far
as this point goes, for preferring a primitive and general plutonic origin for
such a structure to dynamic action resulting from pressure applied on a
' regional ' scale. Of course in either case dynamical action occurs : the
crux of the question is as to the origin of such dynamical action and the
conditions under which it acted. The introduction of the qualifying word
' dynamic ' into the discussion of the structures of the (so called) metamor-
phic rocks, as between the 'mechanical' school on the one side and the
plutonists on the other, leads to nothing but ambiguity.

The habit of speaking of "eyes" in a crystalline rock has however with
some writers in this country transgressed considerably the limits of the above-
quoted definition of Augen-gneiss, till at last it has become difficult to
say that with them even a sheared portion of an intrusive granite, with its
included and modified fragments from the adjacent rocks, may not be com-
prehended under the term.

Orthoclase is probably the embryonic silicate of the terrestrial lithosphere ;
and it is conceivable that in parts of the primordial magma which were under
little or no strain from differential movement of any kind, such felspathic
nuclei as are seen in Augen-gneiss proper may have formed by simple
segregation, that segregation being arrested when differential movements, such
as those which a slow-moving tidal wave would require, began to develope that
metataxic process, which, with the different rates of solidifying of the
minerals of the residual magma, resulted in the structure which we call
' foliation.' Again, if we extend the term ( Augen-gneiss ' so as to include
those gneisses in which lenticular masses of a more basic rock (e.g. amphi-
bolite) occur,f and admit (as we must on general grounds) the possibility of
local and accidental concentration of the heavy bases in the primitive magma,
and bear in mind the greater rapidity with which highly basic slags are known
to solidify, we can explain the more pronounced foliation of the acid matrix
of the gneiss, by the theory here put forward, in such a way as to involve no
mystery, and to conflict with no known laws of nature or observed phenomena.

* Kidges thus formed would have been planed off by subsequent oceanic
tidal action. May we not however here find a clue to the rhyolitic-structure
frequently observed in the included felsitic fragments of the Cambrian
conglomerates ? (See App. ii, Note O. )

t e.g. in the grey gneiss of Elterlein. Credner, op, cit., (6th ed.) Fig. 133,

68 ROCK-METAMOBPHISM.

In such a case as that here cited the feeble foliation of the amphibolite tells
us that the mass was undergoing slow differential movement when the
individualization of its minerals commenced.*

I would suggest one point more, and that is the possibility
that we may come ultimately to associate the feeble foliation
of the fundamental gneiss, where it has not been interfered
with by mountain-building processes, with the earliest solar
tidal waves, and the more pronounced foliation of the archsean
schists with the subsequent lunar tidal waves of the magma.
Even those appearances which simulate ' false bedding ' in
them, on which several recent writers have laid considerable
stress, and the not infrequent interstratification of gneiss
with schists, may perhaps' be partly accounted for in this
way. The subject is worthy at least, I venture to think, of
some attention. In the developement of foliation and its
allied structural characters in the way here indicated in the
fundamental gneisses and schists, the chief difference between
them and similar rocks, in which in some cases perhaps the
general direction of the foliation enables us to connect it
causally with lateral pressure operating in the mountain-
building process, would be that in the former case we should
expect to trace effects due to the pulling-out, and in the
latter effects due to the squeezing -out of the mass. From
our present point of view petrology would seem to be relieved
of the incubus which has been imposed upon it in some
quarters, of having to furnish a theory to account for defor-
mation on a 'regional' scale, qua production of foliation by
pressure acting upon previously solidified rocks. t

* With a still more liberal use of the term ' Augen-gneiss ' we might
include in it cases where, as the outer zone of the lithosphere solidified
through the dissipation of energy by radiation, the tidal movements still
continuing, huge masses, (even ' regional ' masses) of the solidified outer
'crust' would slide over the viscous magma beneath, as the strain produced by
tidal movements in the latter caused huge fractures here and there in the
crust. As the magma grew more and more viscous, it is certain that under
such circumstances large fragments of the thin crust would be torn away,
included in the magma, and transferred in some cases to considerable distances.
This seems the most natural explanation of the striking facts observed by
Lawson in the Laurentian-gneiss, and described by him in his Essay published
by the International Geological Congress, (1888). Facts however such as
those described by Lawson are not new to science. Facts of a similar
nature were described by Macfarlane more than twenty years ago; see his
paper ' On the Geological Formations of Lake Superior,' (Canadian Naturalist,
May, 1867).

t Then as now the tidal action would vary, (within much wider limits
however) with the relative positions of the Sun, the Earth, and the Moon,
the maximum effect being produced when the Moon was ' in meridian.' Pfaff
(Attyem. QeoL ah, ex. Wiss., p. 188, et seq.) has discussed the action of tidal

METATAXIS. 69

In Sec, ii we have regarded the earth from the point
of view of thermal chemistry and physics as passing through
a stage in the history of the evolution of its KOO-/XOS out of its
original non-differentiated xos, when such physical conditions
(temperature and pressure) prevailed universally at its surface
in relation to the fusibility of the different minerals then
being deposited to form a 'crust' as now prevail in the
glacier-zone of a mountain-range relatively to the liquefaction
of ice.

We can recognise a real distinction between granular, vitre-
ous,* and crystalline ice ; and the metatropic and metataxic
changes by which a glacier is developed through the stages of
snow, Firn (neve), and that crystalline-granular form which
constitutes ' glacier-ice ' (with its more or less slabby stratifi-
cation and frequently subordinated banded structure with
white air-charged and blue air-free laminae) have been so fully
studied during the last quarter of a century by some of our
foremost physicists (Helmholtz, Tyndall, Forbes, and others)
that we have little difficulty in referring the peculiar structure
of glacier-ice to unequal rapidity of motion under pressure of
the particles of different parts of the mass of the glacier.
To put the matter more definitely : since equal forces must do
equal work in equal times,

if w = the work done upon a yielding mass,
t = the tenacity of cohesion of the mass,
and d = the amount of change of form produced by the action
of any force p ;

then for any value of p we have in unit of time

w = dxt;

that is to say d varies inversely as t. As in the glacier two
forces are mechanically at work the constant action of the
earth's gravity resulting in a constant strain of the mass
downwards, co-operating with the minor movements produced
by alternate expansion (by day) and contraction (by night) ;
so in the case of the semi-fluid magma out of which the
earliest solid ' crust ' was formed, we have in this section
suggested the action of two mechanical forces the constant
tug of the moon's attraction tending to produce a tidal wave

movements in the magma upon the earliest rind of the Earth, initiating the
first permanent inequalities upon its surface. If the archaean schists (taken as
a whole) represent this first-formed rind, their materials as they accumulated
by precipitation from the heavy atmosphere being bathed through and through
with H 2 O in a highly-superheated condition, we seem to have at once an
explanation both of the frequent recurrence of gneiss (in a subordinate
degree) among the schists as a result of tidal movements, and of those
lithological characters by which they have misled the Neptunists into regard-
ing them as ' sediments.'

* See App. ii, Note F.

70 ROCK-METAMORPHISM.

after it, with a consequent constant strain in the direction of
the motion of that wave, and a series of minor tides due to
the attraction of the more distant sun. In the case of the
glacier we certainly have to take into account liquefaction
under pressure and regelation ; but this we have to do by way
of explaining the 'viscous flow' of the mass. The analogy
drawn above is between the behaviour of a body such as
glacier-ice possessed de facto of the yielding property which
enables it to 'flow/ and that of a viscous mass of rock-
material. The banded structure of both may be explained
by reference to the same physical laws.

For the ' banded structure ' of fundamental rocks whether
on a large scale or that smaller scale which we see in ' foliation '
(in the strictly-limited sense of the latter term) we thus come
to recognise a complex result arising out of the co-operation
of a complex series of causes; the complicated metataxic
changes being connected with such conditions as

(a) inequality of distribution of thermal energy ;

(b) variations in the rate of deposition of the original

materials ;

property,* and in the specific gravity of the
minerals resulting from primary paramorphic
changes ;

all these acting concomitantly with a general dissipation of
energy tending to make the magma approximate nearer and
nearer to the resultant condition of a solid ' crust.' If we
regard any one great class of foliated rocks the gneisses, for
example and the great difference in the degrees to which
their foliation is accentuated, from that feeble degree of folia-
tion which makes it difficult to distinguish- it in a hand-
specimen from a normal granite, to such a marked foliation as
may be seen in some of the gneiss of the Alps,f we can hardly
avoid the conclusion that pressure exerted subsequently by
overlying palaeozoic strata has contributed its quota in some
cases, before the materials of the gneiss were completely
solidified.

If we eliminate the large factor of paramorphic change
(which is essentially chemical) and confine our attention to

* The behaviour of highly acid slags from the blast-furnace as compared
with that of more basic slags from copper and lead furnaces, the former
flowing sluggishly and solidifying slowly, while the latter flow quickly and
harden suddenly, has a most important bearing upon the question of foliation
on a regional scale. This was pointed out more than twenty years ago by
Macfarlane. (Canadian Naturalist, loc. cit., 1864).

t In some of the crystalline rocks of the Engadine (e.g.) composed of
felspar, mica, and quartz, I have observed an unusually distinct foliation.

METATAXIS. 71

metatropic (physical) and metataxic (mechanical) changes, it
is not difficult to draw a parallel between the process by which
we have conceived the vast thickness of crystalline rocks of
the primordial ' crust ' with their coarsely-banded (slabby) and
their finely-banded (foliated) structure to have been developed
out of the earth's original nebulous mass and the process
whereby a crystalline rock-mass of glacier-ice with its banded
structure is known to be derived from the water-vapour
suspended in the present atmosphere. To the geologist who
refuses to look at phenomena of the class generally called
1 geological ' in the light of physical and chemical ideas all
this must read rather like romance than sober science. It is
not for such that it has been written.

V. HYPEEPHOEIC CHANGE.

As a typical process under this head let us consider dolo-
mitization. A limestone containing originally but a small
percentage of carbonate of magnesia is in course of time so
altered in the proportion of its constituents that it becomes
more and more dolomitic, and in some instances is at last
converted into a true dolomite. This must occur in one of two
ways ; either more and more carbonate of magnesia is
deposited within, or carbonate of lime is removed from, the
rock-mass. The former process would be accompanied by an
increase, the latter by a decrease, in bulk of the rock-mass.
Observations in the field afford abundant evidence of decrease
of volume as a concomitant of dolomitization. What has the
chemistry of the laboratory to say to this? Its answer is
twofold :

(1) Experiment shows that water containing free carbonic
acid dissolves a certain amount of carbonate of magnesia, but
that this is insignificant as compared with the amount of
carbonate of lime which the same quantity of equally car-
bonated water can dissolve.*

(2) Thermal chemistry leads us to consider the relative
stabilities of the two carbonates, and reveals to us the fact
that carbonate of lime begins to undergo dissociation with
only a moderate elevation of temperature (having the least
stability of all the carbonates of the alkaline earths), while
carbonate of magnesia is distinguished from most artificially-

* See Roth : Allgem. u. Chem. Geologie, Bd. I, pp. 70 80.

72 EOCK-METAMOEfHISM.

prepared salts of metals by its very great stability. It only
begins to undergo dissociation (under a pressure of one
atmosphere) at temperatures above 300 C, and is only attacked
very feebly at ordinary temperatures by dilute acids.

Thus chemistry settles in a very definite manner the question
as to the modus operandi of dolomitization ; and its answer is
in accord with observations in the field on loss of bulk of the
rock-mass.* A further confirmation of this is found in the
high percentage of carbonate of lime which springs issuing
from dolomitic limestones contain, and the enormous quantity
of calcareous travertine which they deposit. This latter fact
can hardly fail to force itself upon the attention of an observer,
who in the light of chemical ideas will examine the mag-
nificent coast- sections of the magnesian limestone of Durham.
It also explains the formation of dolomitic sandstones out of
the residual dolomitic sand of the rock-mass of which the
Mansfield freestone is a well-marked and well-known instance.
It perhaps accounts, as I suggested in a discussion in Section
C of the British Association four years ago, for the great
quantity of carbonate of lime which is mingled with the fine
siliceous Alpine detritus in the Loss of the Upper Rhine
country : the derivation of this mineral from the regions of
the head-quarters of the Ehine and the dolomitization of the
limestones of those regions being thus seen to be concomitant
phenomena. The comparative readiness with which car-
bonate of lime undergoes dissociation enables us to see how
heat may act as an important factor in dolomitization within
certain limits : lime, one product of dissociation being readily
taken up as a soluble hydrate by pure water, and carbonic acid
(the other product of dissociation) contributing to the solvent
activity of contiguous water within the neighbouring rock.

These principles taken into consideration along with the
great tendency of magnesium to form double salts seem to
throw some light upon the obscure process by which the drusy
cavities of so-called "potato-stones" are lined with a deposit
of pure dolomite or bitter-spar, in the dolomite rocks of
the magnesian limestone of England and in the Bauchwacke
of Thiiringen and other regions of the continent.

Hyperphoric f change as illustrated by dolomitization
includes then all those simple processes in which the character
of the rock is changed by the removal of one or more mineral
constituents from, or the introduction of one or more constituents

* Dwellers on the Magnesian Limestone districts of Durham are from time
to time unpleasantly reminded of this by the Erdfalle which occur as a
consequence of it, cavernous portions of the rock collapsing into brecciated

HYJ>EBPHOEIC CHANGE. 73

into, the rock-mass. The active agencies concerned (partly
chemical, partly physical) vary in different cases. But it will
be seen that hyperphoric change is the essential principle of
such minor processes of ' metamorphism ' as

(1) The conversion in some instances of a vesicular dolerite
into an amygdaloid, especially where the dolerite is intrusive
among limestones.

(2) The gradual formation of gigantic masses of nearly pure
rock-salt (NaCl) as in the Eastern Alps and in the Stassfurt
district. In the latter district the massive saline deposits are
known to reach a thickness of 400 metres, of which the
lowest 200 metres and more are said to consist of pure rock
salt. How was this formed ? Apparently not by original
deposition from concentrated sea-water or mineral- springs,
since in all these other salts are invariably present. The
chemistry of solubilities seems to help us here. We have
only to recollect that the coefficient of solubility of Na Cl in
water is only very slightly affected by temperature (increasing
only from 34 to 36 % for a rise of temperature from to 30C)
while the coefficients of solubility of the other salts contained
in sea-water and in the upper layers of the saline deposits
of Stassfurt and other districts, increase rapidly (that of
sylvine for example from 30 to 38 % and that of Glauber
salt from 5 to nearly 40 %) for an equal elevation of the
temperature of the water. The application of these facts as
involving hyperphoric change is obvious enough, on the
assumption that the temperature of the water in the lower
beds is higher than that of the water in the beds above.

(3) One of the most interesting and best understood
instances of hyperphoric change is the derivation of the
selenite found in clays from pyrites, involving a transfer of
the element sulphur. We have in this case

(i.) introduction of free oxygen in solution in atmos-
pheric water, with a consequent oxidation of the
sulphide of iron into a soluble hydrated sulphate of
that metal, and a transfer of the mineral thus
formed by water ;

(ii.) oxidation of the base into a hydrated peroxide,
which is precipitated, while the acid radicle is
carried away as free sulphuric acid ;
(iii.) a direct chemical reaction, CaC0 3 + H 2 SO 4 = CaS0 4
+ H 2 C0 8 , as soon as the free sulphuric acid comes
into contact with carbonate of lime, the sulphate
of lime thus formed crystallizing as selenite.
I have observed very fine examples of this in the clays
exposed in railway-cuttings near Grantham, and in other
places.

74: ROCK-METAMORPHISM.

As pointed out in the introductory section of this work,
changes such as we have here named ' hyperphoric ' do not
(acting alone) constitute true metamorphism ; yet it must be
admitted that in many cases, and especially in ' contact-meta-
morphism,' they play an important part conjointly with other
agencies. The allotment of a short space to their con-
sideration seems therefore to be justified.

VI. CONTACT-METAMORPHISM.*

This part of our subject has been so thoroughly studied as
to need only a brief consideration here.

The phenomena presented by it exhibit the operation of all
the four principles discussed in the foregoing sections of this
work:

a. Paramorphic change ( ii) ;

b. Metatropic change ( iii) ;

c. Metataxic change ( iv) ;

d. Hyperphoric change ( v).

There is one consideration however involved in the study of
contact-metamorphism, which so far as I know the literature
of the subject does not appear to have had the attention
bestowed upon it which it deserves ; that is the factor of time,
upon which Lyell laid so much stress. We have no right to
assume that all the results, which we can recognise to-day in
any region in which the rocks have undergone the metamor-
phism which can be traced to the contiguity or proximity of
intrusive masses of igneous rock, are traceable directly to its
action. We have to consider also the possibility of further
changes occurring after those forces, which the intrusion of
the igneous mass called into action, ceased to operate. This
will be seen as we proceed. If we try to work out the
probable sequence of changes (in outline) it is not difficult to
conceive of them as falling into several distinct stages.

First stage. Direct effects of heat and pressure. From the
principle of dissipation of energy it follows that the glowing
intrusive mass must continue to impart heat to the neighbouring
rocks until equalization of temperature is reached. The glow-
ing mass having to make room for itself must displace the
neighbouring rocks ; and, as results of this, fracture, crushing,
contortion, and shearing must follow. Also by transmission of

* The terms ' exogenous ' and ' endogenous ' have been used to denote
respectively the appearance of new minerals in the adjacent rocks and in the
intrusive mass. (See Kalkowsky, Lithologie, p. 33).

CONTACT -METAMOBPHISM. 75

the pressure cleavage may be induced in rocks of the outer
zone of the region affected. The proportion of these will vary
in different instances according to the nature and condition of
the rocks acted upon ; the more rigid rocks suffering more
crushing and fracture, those in a more pasty condition suffering
more shearing. There will also be some variation in these
results in different parts, according to the direction in which
the pressure is applied to them. In every case mechanical
force is transformed into heat, and this is added to that received
by conduction from the glowing intrusive mass, together with
that produced by the friction of the intrusive mass against the
rocks into which it is intruded. The direct effects of all this
would be mainly metataxic and metatropic in their nature.
As metatropic results we may note such as vitrification, fritting
and baking, both of the adjacent rocks and the fragments
caught up and enclosed in the molten mass, the results being
often most marked in the latter. In many cases they are
melted directly into a glass, in others crushed and recemented
by a glassy cement. Some are molten wholly or on the
exterior into slags, of which latter I have a good example
which I obtained some years ago from the ash-cone of the
Pulvermaar in the Eifel ; others are rendered porcellanic in
their hardness, or acquire in some instances a columnar
structure (as I have known lumps of fire-clay do in the
burning pit-heaps of the coal-country) ; others again are
coloured or bleached (according to the nature of their
accessory materials) by baking. According to Lehmann cases
occur in which the enclosed masses are (wholly or in parb) so
completely liquefied as to allow diffusion of their material
through the surrounding magma, reacting upon it to change
locally its actual composition. * In the Vorder-Eifel and
other parts of the Continent fragments of clay-slate and
grauwacke have been observed baked on the exterior ; frag-
ments of mica-schist, quartz, and gneiss are found covered
over with a vitreous crust ; fragments of sericite- schist (in the
particular locality Naurod near Wiesbaden) have only their
layers of sericite and chlorite vitrified. Even the fragments
caught up from older igneous masses, through which (or upon
which) later flows find their way, are not exempt from this
kind of metatropic change ; as in the case recorded in the Isle
of Ischia, where fragments of an older trachyte-flow are found
included in a later flow and converted into slag. Fragments

* A very striking instance of this has been described by Macfarlane in the
basic intrusive rocks of the Huronian Series of Lake Superior. In some
places the included granite fragments have been so completely dissolved by
the basic magma as to convert the rock locally by shearing into a ' siliceous
slate rock.' (Canadian Naturalist, May, 1867.)

76 EOCK-METAMOBPHISM.

of limestone are converted into marble, but not always wholly
so ; the larger fragments retaining in some cases an unmeta-
morphosed core, which gives a direct clue to their parent-rock.
As remarkable examples of this we may cite the fragments of
Silurian limestone at Escabar in the Pyrenees described some
years ago by Zirkel, the fragments of Trias limestone in the
augite-porphyry of the Grodnerthal described by Eichthofen.

Similar in their nature are the changes produced upon the
neighbouring rocks, so far as the action of dry heat is concerned.
Sandstones are decolorised and often fritted to a glistening
enamel-like or porcellanic mass ; in other cases where the
cement is of a calcareo-argillaceous nature this is melted into
a glass ; clay and marl are converted into porcellanite or brick,
with marked change of colour in many cases ; tuffs and
phonolites are so far vitrified as to acquire a character
resembling that of obsidian ; brown-coal is altered into seam-
coal or anthracite, and these in other cases into a substance
more resembling graphite, while in others (probably under less
pressure) the coal is converted into coke,* the various shades
of metatropic change of brown-coal into anthracite, carbon-
glance, bituminous coal, and black-coal being observed in some
cases in the same section through several metres of the mass ;
a prismatic structure is developed, not only in clays and
marls, but even in sandstones, in brown-coal, in seam-coal, in
dolomite ; limestones are altered into crystalline marble, often
with complete effacernent of their stratification and even of all
traces of their fossils ; the finer varieties of grauwacke and its
associated shales are converted into hornstone, as in the
classical region of the Brocken. Of these metatropic changes
by the action of dry heat under pressure, thab of the formation
of marble has been experimentally verified years ago by
G. Rose, and more recently by Eichthofen and others.

The amount of metataxic change which the neighbouring
rocks may undergo will vary greatly according to the nature
of the rock upon which the force is brought to bear ; and it
would be difficult to deny that along with cleavage as a direct
result of pressure in some of the rocks, especially in the more
argillaceous, we may have a crude sort of foliation developed
in some cases by shearing, pari passu with fluxion-structure
and a certain degree even of foliation in the intrusive mass

* Marked instances of this are known in the Meissen district ; and several
years ago I noticed a fine example in the Museum of the Reichsanstalt at
Vienna of the local conversion of seam-coal into coke by a basalt dyke which
traverses it. These are of course instances of 'destructive distillation.' In
Ayrshire, graphite is said to result from the action of intrusive masses of
dolerite upon the seams of coal ; probably a dissociation-product by contact-
action of the hydro-carbons distilled over from the coal.

COTSTACT-METAMOBPHISM. 77

itself.* In this way the schistosity induced in contact -met a-
morphism may in part be accounted for. In other cases
however the deformation of rocks would proceed rather by
way of contortion, or of fracture and crushing, where the
rocks were sufficiently rigid ; and the importance of this is
perhaps chiefly to be seen in the favourable conditions thus
brought about for the operation of the agency next to be
considered.

Second Stage. Circulation of superheated water between the
intrusive mass and the adjacent rocks. In ordinary lava-flows
the water included in the outpoured magma, being relieved to
a great extent from pressure, escapes principally as steam ;
but in the case of the plutonic rocks there is no difficulty in
understanding the continued action for a lengthened period of
time of superheated water, as in M. Daubree's experiments, in
which the water was maintained at a temperature of 400C
for several weeks. This circulation of superheated water in
both directions from the igneous mass into the adjacent
rocks, and conversely must result in bringing its enormously
increased solvent power into play upon the mineral con-
stituents of both sets of rocks, and especially upon the silicates.
Hyperphoric change is thus brought about continuously, so
long as the high temperature is maintained and there remain
in the original rocks minerals of lower degrees of stability
(under such conditions) capable of being taken up by the
water and of reacting upon one another to produce minerals
of a higher degree of stability. This kind of transfer of
material would seem to take place (from foregoing con-
siderations) most extensively by way of solutions of the
alkaline silicates, though not confined to these. The tem-
perature of the glowing mass being higher than that of the
neighbouring rocks, the solvent action of superheated water
must no doubt be exerted in the highest degree as a general
rule upon the minerals of the igneous mass ; but it can hardly
be doubted though this does not appear to have been very
generally recognized that there is also a converse and sub-
sidiary transfer of mineral matter from the adjacent rocks into
the more superficial portions of the igneous mass, resulting in
a certain amount of paramorphic change. t This will be seen

* Dr. Barrels ('Modifications et Transformations des Granulites du
Morbihan' ; Ann. de la Soc. Geol. du Nord, T. xv., p. 1, et seq.) has described
such schistosity along the zone of contact of the granulites with the Cambrian
slates. As examples of the same action of shearing in basic rocks, those
occurring in the dolerites of the Huronian described by Macfarlane (op. cit.)
as ' greenstone slate,' and those more recently described by the officers of the
G. Suryeyof Scotland, (Q.J.G.S., vol. xliv., pp. 391-5.) may be cited.

t We must look in all such cases beyond the mere mechanical circulation of
water to the action of aqueous diffusion ; the tendency, that is to say, of any

UNIVERSITY

78 ROCK-METAMOKPHISM.

to throw some light upon those cases in which it has been
alleged that a gradual transition from plutonic into clastic
rocks has been observed. No clearer case of this kind of
reaction (through the medium of superheated water) of the
original sedimentary rock upon the intrusive igneous rock has
perhaps ever been noted than that described by Von Cotta as
observed by him in the year 1849, in the marble- quarries
above Predazzo, where the junction of the granite with the
limestone (altered into marble along the zone of contact) is
extremely sharp and well denned. Apophytic processes of
the granite-mass have been injected in places into the dolo-
mitic limestone, and as these (in one instance at least) are
traced further and further from the granite massif they
become more and more talcose, and ultimately pass into a
true serpentine dyke in the midst of the limestone.* Here,
where the proportion of mass is all in favour of the influence
of the limestone by hydro-thermal action upon the granite, we
find that action predominating, furnishing an extreme case of
alteration of the igneous rock. The same author noted the
partial serpentinization of granite injected into serpentine
itself at Waldheim in Saxony.

These recorded observations of Von Cotta's are supported by those of Prof.
Heim on the quartz -porphyry of Windgalle in the Central Alps, ( Mech. d.
Gebirgsbildung, Bd. i, p. 37,) the general description of which by that author
(op. cit., p. 36,) would apply to the Bozen porphyries as I know them. The
analysis which he has given of the normal rock shows that its chemical
composition approximates closely to that of a granite of medium composition
with a rather excessive percentage of the alkalies. "In certain modifications
[of the porphyry] the ground-mass contains irregular elongated masses (in
some cases more than 2cm. long and 1cm. thick) of streaky dark olive-green
serpentine. In thin sections it is seen that these serpentine -masses are often
inter-plaited (verflochten) at their boundaries in remarkable dendritic forms
with the ground-mass. Strips of the ground-mass are often floated as

inclusions in the serpentine These modifications of the porphyry are

quite local. " It is not easy to understand how the MgO required for this
serpentinization could be introduced otherwise than by the solvent action of
water at rather high temperature and pressure circulating between the intrusive
mass and the adjacent rocks. This view is certainly in harmony with the
statement of Kalkowsky (Lithologie, p. 72). " Endogene Contacterscheinungen
sind bisher [in quartz-porphyries] nur in geringem Masse beobachtet worden,
es gehort dazu namentlich das Feinerwerden des Kornes, das Wegbleiben der
Einsprenglinge." The same writer notes (op. cit., p. 65,) tourmaline and (less
frequently) andalusite, hornblende, garnet as (probably) endogenous products
of contact-metamorphism in granite near the boundaries of the massif and in
its apophytic extensions.

substance dissolved in water to diffuse itself equally throughout the mass. In
the water-logged condition therefore of such rocks, whatever is taken up in
solution from one part must be distributed through the water until it enters
into new combinations ; thus the nature of the endogenous changes will be
affected by the lithological environment of the intrusive mass.
* See App. ii, Note N.

CONTACT-METAMORPHISM. 79

Even the more basic intrusive masses may undergo such endogenous
alteration as to develope near their junction-planes such minerals as tour-
maline, pleonast, corundum, andalusite, and others, as seen in the recently
investigated instance of the diorite intruded into phyllites at Klausen, (Tirol).
Kalkowsky, (op. cit., p. 98).

The late lamented Prof. Carvill Lewis has described the alteration of an
ultra-basic peridotite (Kimberlite) in South Africa, in proximity to car-
boniferous shales, with the endogenous formation of secondary minerals
including diamond. That an important part has been played by the excess of
the bases of iron and magnesium set free in the serpentinization of olivine,
aided by heat and pressure, in the reduction of the carbonaceous material, can
hardly be doubted. (See Brit. Assoc. Keports, 1886-7).

As another instance of the influence of mass in determining the final
results, may be mentioned the occurrence (which I have lately observed)
of strongly dichroic hornblende near the junction-plane in the fine-grained
felsite which is injected into the hornblendic granite of Mount Sorrel,
with which I am pretty familiar from field-observations. In such cases of
transfer by the agency of aqueous solutions the mass which supplies the
material must be commensurate with the time required for bringing about the
result, for the simple reason that, as soon as the materials which furnish the
'antecedents' of a chemical reaction fail, the 'consequents' must fail also. It
is not until we realise the full meaning of this that we can appreciate
rationally the importance of time as a factor in all such changes, or open our
minds to the full extent of the cumulative results which extremely dilute
aqueous solutions are capable of producing. Compare the statement of the
officers of the Scotch Survey as to the absorption of calcareous matter (Q.J.G-.S.
xliv., p. 410).

With regard to the changes of composition and structure which eruptive
rocks undergo in their outer zones (salbandes) in contact with the neighbour-
ing rocks (modifications endomorphiques,') Barrois remarks (op. cit., p. 2) that
the basic eruptive rocks have principally furnished a great mumber of examples
of these modifications ; but the acid rocks do not escape the law, examples
having been described by Lossen in the Hartz, by Lehmann in Saxony, and
by himself in Einisterre and at Morbihan.

We must be prepared, from what we know of diffusion of
bodies by solution in water, to recognize such influence in
different and minor degrees. The ideas put forward in ii (/3),
as to the influence of salts dissolved in sea-water upon sub-
marine lava-flows, in promoting secondary paramorphic changes,
apply also (mutatis mutandis) here. It is impossible then to
deny that many and various phases of secondary paramorphism
may result in the outer zones of an intrusive igneous mass ;
and when this is recognized, along with the crude foliation
often observed in such zones near the plane of junction as the
result of metataxic change in the unconsolidated magma,
not only bases of the alkalies alkaline earths and magnesia,
but even alumina* and heavier metallic bases being no doubt
in some instances introduced to a slight extent, we have got
a long way towards the explanation of those observed
phenomena, which have led many observers to argue in favour
of such extreme views of regional metamorphism as would

* It is as a sulphate or a hydrous chloride that Al is mainly capable of
transfer in solution, (cf. Roth. : op. cit., p. 112).

80 BOCK-METAMOEPHISM.

persuade us that we may find in many districts all degrees of
metamorphic change, proceeding gradually from the unaltered
clastic sedimentary rocks at one end of the series, to normal
granite (as so much boiled-down sandstone) at the other.*
If this is admitted, there seems to be a poor foundation in
fact left for extreme views of regional metamorphism ; and as
we have before disposed of the argument from the presence of
clastic materials in crystalline rocks (result of crushing) this
beautifully-elaborated but empirical theory tumbles down like
a house built of cards.

The formation of exogenous ' contact-minerals ' in the
rocks adjacent to the intrusive mass by reactions resulting
from the hyperphoric introduction into them of mineral -matter
in solution from the igneous mass have been long and carefully
studied. Among these are to be noted especially the intro-
duction of silicates in solution to form in limestones as they
are metamorphosed into marble, garnet, vesuvian, wollastonite,
skapolite (wernerite), prehnite, epidote, minerals containing a
large percentage of lime ; occasionally others containing a
smaller percentage of lime, such as hornblende, tremolite ;
spinellf (in which MgO takes the place of CaO), titanite,
fluorspar, and some micas. The Predazzo region has long
been renowned among continental petrologists for the facilities
it offers for the study of such contact-minerals and their
derivative relation to the limestones and the igneous masses
intruded into them. Near Monzoni a pretty general sequence
of zones has been worked out, characterized in succession by
garnet, augite, serpentine, brucite, I as results of mineral-changes

* See in particular Prof. Green's idealized sketch, Physical Geology, p. 453.
There is a dangerous fascination about such graphic expressions of general
ideas : we are liable to be misled by their very attractiveness and apparently
to think that as it looks as if it were so, it must be so ; and thus the critical
faculty is in danger of having its edge taken of. Take again the concrete
instance brought forward by Mr. Green (fig. 126, p. 401). What is there in it
that cannot be explained as the obtrusion aud overfolding of a part of the
archsean crust together with the oldest Cambrian strata even to the ' bedding '
of the ' crystalline ' rocks ? f See App. ii, Note Q.

A point of considerable theoretical interest seems to receive illustration
from this fact. Taking these four minerals in the order in which they are
enumerated we seem to have a definite order of variation in the genesis of the
exogenous minerals, those portions of the metamorphosed zone which are in
immediate contact with the augite -syenite having been presumably raised to
the highest temperature, thus :

1. Garnet (presumably) Caa A1 2 Sis Oia, gives bases : Si0 2 = 4 : 3.

2. Augite (presumably) Ca Mg Si2 Oe, gives bases : SiC>2 = 2 : 2.

3. Serpentine, H4 Mg 3 Si 2 Og gives bases : SiC>2 = 5 : 2.

4. Brucite, Mg (OH) 2 .

With the exception of garnet, in which some of the A1 2 0$ may be present
as aluminate (cf. App. ii, Note Q), we note a general diminution in the
proportion of SiO 2 taken up, with a corresponding increase of readiness to
take up ' water of constitution,' with lowering of temperature,

CONTACT-METAMORPHISM. 81

effected by -silicates introduced in solution from the intruded
augite-syenite. The formation of garnet in this way is a
matter of observation in numerous localities. In southern
Norway other contact-minerals have been carefully studied,
portions of the Silurian strata having been, as it appears,
metamorphosed completely by the granite and its numerous
apophytic intrusions. Not only are limestones converted
into marble, but calcareous slates also into a calcareous
/hornstone or a crystalline schistose rock, some layers of
which contain garnet and epidote, while gneissic mixtures
of mica, pyroxene, quartz and felspar, have been developed.
Even the cement-stones are described as converted partly into
garnet and vesuvian, and graptolite-shale into chiastolite-slate.
In many cases the organic remains of the original rock are
found hard by the newly-formed minerals. If these southern
Norwegian phenomena can be thus explained as the result
of contact-metamorphism, it is difficult to see how they can be
quoted in favour of extreme theories of regional metamorphism.
Very noteworthy are the results produced by the intrusion
of igneous masses into rocks which have been converted by
previous metataxic change into slates. Here we have often a
regular sequence of phenomena, for exact knowledge of which
we are indebted largely to Eosenbusch. The normal and
progressive order of change observed is, according to that
distinguished petrologist, (1) the appearance of small knotty
concretionary bodies in the generally unaltered slates; (2)
these increase in number and quality as we approach nearer
to the granite or other central intrusive mass, while pari passu
with this the slate becomes more intensely crystalline, until it
assumes almost a schistose character ; (3) as this schistosity is
more and more developed the knotty appearances diminish
and ultimately disappear altogether, the slates passing into a
thoroughly crystalline rock with a complete schistose structure
at the zone of contact. These phases of structure occupy in
some cases altogether zones of rock whose thickness is
measured by hundreds of metres. Kosenbusch has described
cases in the Vosges, in which he distinguishes

a. the zone of Knotenthonschief er ;

b. the zone of Kiiotenglimmerschiefer ;

c. the zone of hornstone and andalusite-hornstone.

Similiar results of contact-metamorphism have been worked
out in the Erzgebirge by officers of the Saxon Survey, where
a similar increase of intensity of alteration towards the
granite is seen in the phyllites, quite independent of the
original phyllitic structure, furnishing well-marked zones of

82 ROCK-METAMOBPHISM.

a. Fruchtschiefer with unaltered ground-mass of slate ;
6. Fruchtschiefer with a crystalline ground-mass of slate ;

c. schistose micaceous rock ;

d. audalusite-mica rock.

F. E. Miiller has described similar metamorphic stages in
the neighbourhood of granite in Thiiringen. Even gneiss
and mica-schists undergo contact-rnetamorphism, with the
developement in them of such new minerals as sillirnanite,
andalusite, and tourmaline.

The developement of felspar, sillimanite and biotite in the
Lower Silurian sericitic sandstones of Morbihan in Brittany
has been described by Barrois, as also an outer contact-zone
of biotite-bearing quartzite.

The comparative and analytical studies of Rosenbusch and
others have led to the conclusion that in the contact meta-
morphism of some slates, in the zone near the graoite, the
change consists in a molecular re-arrangement of the original
constituents of the slates with a simultaneous taking-up of
water of constitution (metataxic and metatropic changes)
rather than in the introduction of new mineral-substances.
ID the exogenous developement however of such minerals as
tourmaline, as described by Allport (op. supra cit.), new mineral-
matter must have been introduced.

For the facts cited above I am largely indebted to the latest
edition of Credner's Geology. Many others may be gleaned
from works and memoirs published in this country.

Third stage. Changes following upon the cooling of the
igneous intrusive mass. So far as my reading goes, these have
received little consideration. Yet a little reflection will show
that some further changes may occur in many cases. The
igneous mass having in the course of its intrusion made a
space for itself to occupy continues to occupy that space so
long as its high temperature is maintained; but as its tem-
perature gradually falls it suffers loss of bulk. The necessary
result of this is either (a) a considerable widening of the
junction-plane, or (b) a settling-down of the displaced rocks
to fill the space left unoccupied. The latter may leave its
mark in Ausiueichungsclivage* in the adjoining rocks, while a
certain amount of contortion or fracture with perhaps faulting
on no very large scale may occur in the rocks nearer the
junction-plane. If these results do not at once follow, the
junction-plane is meanwhile widened. In cases where the
igneous mass has become welded to the adjacent rock, the
latter may part asunder at places of greatest structural

* ' Ausweichung ' (lit.) lateral yielding. See Hei n ; Mech. der Gtbirys-
bttdung. Bonney's term ' strain-slip ' is a very good English equivalent.

CONTACT-METAMORPHISM. 83

weakness.* In either case it is easy to see that increased
facilities are given for the access of water from above, bringing
with it in solution either atmospheric gases or mineral
substances taken up in its passage through superjacent rock-
masses. The access of this water must lead in many cases to
a series of secondary paramorphic changes both in the surface
portions of the cooled igneous mass and still more in the
adjacent (and in some instances partly disintegrated) rocks of
the already metamorphosed zone. The water itself would
aid in the further disintegration of the rocks to which it thus
found freer access. It is possible that much of the water-of-
constitution noticed by Rosenbusch may have been taken up
under such conditions, which are more favourable to it than
conditions of high temperature. A certain loosening of the
structure of the rock from the causes here indicated may, along
with the freer access of water, help to explain the ' knotty '
texture developed in them. If such changes went on for a
length of time, a great deal of work might be done to prepare
the way for such metatropic and metataxic changes as would
result in developing to a greater degree the schistosity of the
rocks of the contact-zone under the influence of pressure
exerted at a later period in great earth-movements upon the
region.! Many of the phenomena presented in regions of
contact-metainorphism may thus be found susceptible of a
more rational explanation than the hypothesis of direct defor-
mation of a solid rigid rock by pressure. The work done by
pressure would in such a case consist rather in re-forming than
in deforming the rock.

The ultimate result would be to give us a pseudo-schist, by the welding and
shearing under pressure of the loose materials in which long-continued hydro-
chemical action had previously wrought such secondary paramorphic changes
as have been by some writers attributed to the pressure itself. In fact the result
wjuld be much the same as the action of pressure and shearing upcn a mass
of fault-de"bris, where the adjoining rocks could furnish the required mineral
constituents, the true history probably of some pseudo-schists which take a
dyke-like form.

* This I believe to be the explanation of the peculiar marking of many
marbles, such as the Devonian red marbles with their included corals. The
cracks (filled subsequently with secondary calcite) are so disposed as to suggest
the rock having been torn asunder much in the same way as the marginal
transverse crevasses of a glacier are formed, where the margin of the
glacier is held by the rock of the valley-side, while the more central portions
of the mass move onwards. I am also inclined to think that the peculiar
structure of the ' Marbre de Plestin ' of Barrels, (Am. S. G. du Nord
T. xv, PI. iv.) may be explained in this way.

t There is a third result possible. The contraction due to the cooling of the
outer and more elevated parts of the intrusive mass might, by the pressure of
that mass and the adjacent rocks, cause a welling-up of portions of the still
fluid magma beneath. Such may be the possible explanation (as its relation to
the granitic massif would suggest) of the occurrence of aplite so frequently
near the junction in the intrusive rocks of Brittany. (See Barrois, op. cit.)

84 ROCK-METAMORPHISM.

Prof. Boyd Dawkins has lately drawn my attention to some interesting
examples (in the Museum of Owens College) of mineral -change in the felspar
of the granite contiguous to the principal lode of the Foxdale mine in the Isle
of Man, which he has investigated. Near the lode, on the side on which a
vein of spathic iron-ore occurs, a good deal of the felspar has acquired a green
colour. Under the microscope the plagioclase appears to be quite fresh and
unaltered, while the orthoclase has suffered two different modifications ;
(1) ordinary kaolinization with no green colouration and the separation-out of
much SiC>2, (2) alteration to a green mineral, in which the separation-out of
SiOa appears to have taken place much more sparingly. These facts seemed
to me to suggest that traces of iron-salts had been diffused in solution through
the granite-zone on that side, and that silicate of iron had been formed by an
ordinary chemical reaction between the salts of iron and silicate of potassium
of the orthoclase, the stronger acid of the iron-salt taking up the potassium,
which thus was carried away in solution, with a simultaneous combination of
iron and silica. An analysis of some of this green mineral, which I made in
the chemical laboratory of Owens College, showed that this was indeed the
case. Secondary (endogenous) paramorphism has occurred here apparently
independently of pressure and high temperature.

Many examples of metamorphism in regions contiguous to
junction-planes might be cited to which the operation of the
causes tending to change in this third stage would seem to
apply. For the British area I shall content myself here with
reference to one in the Cornish peninsula lately described by
so careful a petrologist as Mr. Teall,* and to the published
papers of Mr. S. Allport.

After a careful study of Mr. Teall's admirable paper I must
say that the facts which he has described seem to me to point
very strongly to the deformation of the mineral structure of the
metamorphosed portions of the igneous rock by free access of
water along the junction-plane, and perhaps through shrinkage-
cracks followed by a re-formation of the rock of those parts
less deformed into a flaser-gabbro and of those more deformed
into a gabbro-schist by the subsequent action of the pressure
and shearing which accompanied those great earth-movements
of which the district seems to furnish abundant evidence. The
nature of the mineral-changes indicated, e.g., the diminution
in the percentage of silica and the taking-up of water-of-
constitution, together with a slight increase of percentages of
some of the accessory mineral-ingredients (see Eammelsberg's
analyses) in the alteration of labradorite into saussurite seem
to point to the action of water ; while the fact that some parts
of the rock are stated to be ' brecciated ' while other parts are
foliated, seems to illustrate the different ways in which the
undeformed and the deformed portions of the rock were
affected when pressure acted subsequently upon them. When
these things are considered, the case of the Lizard gabbros

* "The Metamorphism of the Lizard Gabbros," Geol. Mag., November,
1886.

CONTACT-METAMOEPHISM. 85

scarcely seems to give such support to the theory of ' regional
metamorphism ' as the accomplished author of the paper
quoted appears to imagine.

With this we may compare the most instructive instance of the quartz-
porphyry of Windgalle in the Central Alps described by Prof. Heim (Mech.
der GebirysbUdung, Bd. i, pp. 34-39). At the junction of the pre- carboniferous
intrusive porphyry with the older Verrucano-strata there occurs,locally a rock
of such a character that it may be regarded petrographically as a transition-
rock ('Uebergangsgestein ') between the two. " The clear grey ground-mass
contains still small rose-red felspar crystals and pellucid quartz-grains as the
porphyry does, but they are manifestly crushed, and sericile occurs in considerable
proportion. A definite boundary (Gesteinsgrenze) in the direction of the
slaty rocks, described as of Verrucano character, is not to be found." Prof.
Heim records his own impression that crushing (Quetschung) has driven
the petrographical limit between the two rock series some distance (ein Stuck
weit) into the originally massive porphyry ; and he rejects the notion that
either series is a metamorphosed representative of the other. I would suggest
that in this instance also we may have the combined results of two distinct
processes, pressure operating during the later mountain-building movements
(of which such overwhelming evidence is furnished) upon the intermingled
materials produced along the junction- plane in the disintegration of the rocks
of both series in this third and latest stage of contact -metamorphism, which
had resulted from the freer access of water, and of which the incipient stage ; ,
is represented by the kaolinization of the outer crust of the intrusive mass, j
(p. 35.)

Lehmann (Alt-Kryst. Schiefer-gesteine, Taf. v. vi.j shows us how pressure
produces a metataxic change (not uncommon) in both granitic and clay-slate
materials whether fragments of slate enclosed in the granite, or granite
injected into the slate inducing cleavage in the slate-fragments and
converting the granite locally into 'phyllite-gneiss,' without (except in one
doubtful case, Taf. v. fig. 3) inducing any paramorphic change of either rock,
or even obliterating in the slightest degree the junction of the two. On Taf.
v. (fig. 2) mechanical deformation of the quartz is to be noted as apparently
the main factor in bringing on the phyllitic structure. Here again it seems
not at all unlikely that the quartz, having been perhaps in a hydrated
condition, has passed through the 'critical state' (supra, pages 48-49.) Taf. xx.
of the same work exhibits a fine section of gabbro, in which the flaser-
structure and even a 'diinn-schiefrige' structure are seen to be so related to
the junction-plane and to the shrinkage-cracks that the explanation suggested
in the text of this work (pp. 82-83) seems directly applicable to them.

To the cases cited here may be added the following remarkable instances from
Allport's masterly papers :

1. The fact, which he mentions and shows in an illustration (Q.J.G.S. vol.
xxxii. fig. 4), of the crystalline tourmaline, in the tourmaline-schists in
contiguity with the granite, adapting itself to the form of the previously
crystallized quartz, seems to suggest crystallization of both in the 'dry way,'
the melting-point of tourmaline being well within the range of furnace and
blow-pipe temperatures. (Hammelsberg, op. cit., p. 538).

2. What resembles a Tleckschiefer' is described by Allport, (ibid. fig. 3)
as occurring in the slate about 15 feet from the granite, "the granular parts
consisting of numerous flakes of mica thickly crowded together, and some-
what elongated in the direction of the [incipient] foliation," (pp. 409, 10).
The observation on page 408 of the same paper, that "decided foliation is
restricted to the immediate vicinity of the granite, and appears to replace the
original lamination of the fine sedimentary material" is of some importance,
as suggesting the necessity for caution in drawing inferences from observa-
tions in cases in which a continuous section is not seen.

Ob ROCK-METAMOKPHISM.

3. Allport's observations on the phenomena of contact-nietamorphism
(Ibid. pp. 407-10,) seem to run fairly parallel with those of such foreign
workers as are cited in this work, (pp. 81-82). He notes the developement of new
minerals (quartz, mica, tourmaline) in the contact-zone of the slates, and the
conversion of previously-cleaved clay-slate into a crystalline foliated rock
(mica-schist, tourmaline-schist), the arrangement of the new minerals appear-
ing to follow the original lamination of the fine sedimentary material. The
intruded granite is also schorlaceous. He also describes a case of develope-
ment of crystals of orthoclase felspar, as well as quartz and mica, in the
altered Silurian slates of Wexford, in immediate contiguity with the granite
(Ibid. fig. ii), thus giving us a gneiss as an alteration -product. Yet in spite of
such local phases of metamorphism in the contact-zone, the distinction
between the granite and the metamorphosed rocks seems to him absolute and
incontestable.

4. He notes (Ibid, p. 41 2 ) that, outside the contact-zone of schists,
"indurated slates traversed by quartz-veins form a wide zone, and beyond
these the rock is ordinary clay-slate." With this we may compare Lehmann's
example of Knotenschiefer ( Altkr. Gest. Tafel xxviii, fig. 2} which that author
describes as showing "the splitting of the original slate visibly into small bits,
between which quartz and biotite have separated-out, the secondary quartz
(Neubildungen) forming in places wider quartz-layers." All this, in the one
case and in the other, looks very like the kind of work we should expect to be
done in the third stage of contact-metamorphism (supra, pp. 82-83). The
alteration described by Allport (loc. cit.) of crystalline minerals into 'chloritic
pseudomorphs ' consisting of a green substance which exhibits no bright
colours, must also, I think, be assigned to the same stage.

The alleged metamorphic origin of granite. Prestwich summarizes the
evidence in favour of this doctrine (Geology, vol. i. p. 429J as follows :

(1) The later crystallization of the quartz, though quartz has a higher

fusion-temperature than the other mineral-constituents of granite.

(2) The lower density of the varieties obtained by artificial fusion of

quartz, as compared with the density of the quartz of granite.

These are old arguments, but the reiteration of them by so eminent an
authority at this time of day demands some attention. What do they amount
to?

(i.) Crystalline quartz certainly fuses at a higher temperature (that of the
oxy-hydrogen blow-pipe) than orthoclase and mica do, since they fuse at
ordinary blow-pipe temperature. But here we are dealing with perfectly
anhydrous Si 0%. Given traces of H>2 O in the original magma, as
crystallization of the anhydrous orthoclase and mica proceeded, the Ha O
would be more and more concentrated in the residual Si O 2 , which is simply
the excess of Si O 2 over the proportion required as the equivalent proportion
for the bases present. The concentration of Ha O in this residual SiO 2 would
make it more and more hydrated with a consequent lowering o/ its fusion-
temperature (as my experiments given in App. i. seem to show). This water
by molecular separation from the Si O 2 would collect in minute bubbles (the
true origin perhaps of the minute bubbles included in the cavities) ; and with
this dehydration of the Si O 2 , together with the removal by it in solution of
traces of fluxing-materials (such as Sorby observed in some of the fluid-
inclusions of the quartz of granite), crystallization of the Si O 2 would
advance.

(ii.) The argument from relative densities is fallacious, since the pressure
conditions, under which the experimental evidence referred to is obtained, are
not the same as those which obtained in the crystallization of the quartz
found in granite.

In considering this point we have a further question to ask in connexion
with the work referred to in the foot-note on p. 80. What right has the
author to assert generally, [with reference to fig. 231, on page 669 of his work]
that "the rock [granite] shades off invisibly into foliated schists, and these

CONTACT-METAMORPHISM. 87

melt away in the unaltered rock."? Rather extensive observation of the
succession to be seen in actual mountain-chains is adverse to such a general
statement; and Heim's sections across the Alps (Mechanismus der Ge-
ben/sbildung}, to which he refers, can scarcely be said to bear it out.
Not] ling can be more decisive against such a view than the results of the
work of Allport, "On the Rocks surrounding the Land's End Mass of
Granite" (Q.J.G.S., vol. xxxii.), and that contained in the more recent
papers by Dr. Callaway, (i.) "On the Granitic and Schistose Rocks of
Northern Donegal," (Q.J.G.S., vol. xli.) ; (ii.) "On the Alleged Conversion of
Crystalline Schists into Igneous Rocks in County Galway," (Q.J.G.S., vol.
xliii.) ; which have disposed of the views maintained in some quarters as to the
metamorphic origin of the granite in those districts, by establishing its
intrusive origin. Thus in one instance after another that view is found on
closer examination to be based on insufficiency of observation. Most
important in this connexion is the work of Dr. Ch. Barrois on the
'Modifications et Transformations des Granulites du Morbihan (Granites a 2
micas),' Annales de la Societe gologique du Nord, t. xv.

GENEEAL EEMAEKS.

Much of the obscurity which has hitherto hung over the
subject of ' Metamorphisni ' has been more in the vague and
indefinite not to say at times shifty use of the terms
employed in discussing the phenomena, rather than in the
phenomena themselves.* It ought to be possible to have as
sharply-defined a nomenclature in physical geology as in
biology or chemistry : indeed, without such an armamentarium
it is not easy to recognise its claim to a position among the
exact sciences; it must continue to hold the secondary
position of a mere descriptive study, such as that which
"Natural History" occupies in comparison with modern
Biology. Some limitations for example must be imposed upon
the use of those much-abused terms ' schist ' and ' slate ' ; and
there is no better definition of them perhaps than that which
Prof. Jukes laid down years ago based on Sedgwick's definition
of the terms lamination, cleavage, and foliation. Of the
German word ' Schiefer 't there seems to be an almost
unlimited variety of uses ; but for my part the difficulty is not
such a real one as that of the word * schist,' since the use of
the adjectival form ' schieferig ' on the one hand, and on the

* The high-sounding term ' metamorphism ' (with its cognate forms) has
served too often as a sort of ball-and-socket joint, upon which the logical
lever may be made to turn any way you please according to the exigencies of
the argument. It would be a gain to Geological Science if this group of words
were to drop out of the nomenclature altogether.

t Nothing can be more treacherous than this word to a reader possessed of
only a slight acquaintance with the German language. Without very careful
reading a German writer may easily be understood to speak of schists when all
the time he means slates or even shales.

88 EOCK-METAMOEPHISM.

other the readiness with which the German language lends
itself to word-building, makes it pretty clear generally to a
careful reader what a writer intends, whether (e.g.) he is
speaking of a true schist, of a phyllite, of a slate, or the
purely mechanically-imposed structural features of a laminated
sandstone or shale. Extensive observation of the last-men-
tioned varieties of rock in this country and in Germany makes
it difficult to deny that in some of these the coating of micaceous
spangles on the planes of lamination may have resulted in
some instances from subsequent change of some kind by the
action of infiltrating waters ; yet we should never dream of
calling such a rock a ' schist.' Even when such laminated
sandstones have undergone such an amount of baking in
contact-metamorphism as to cause an incipient interfusion of
the mica and the other materials of the rock, while the clastic
structure of the rock remains conspicuous, it can only be by
an abuse of the term that such a rock can be called a ' schist.'
Mere etymology is in this case (as in many another in exact
science) worse than worthless as a guide. No more are we
justified in applying the term ' schist ' or * schistosity ' to the
peripheral laminated structure developed in the weathering of
some of the dolomitic concretionary mudstones of the Durham
coast ; or in the weathering of blocks of basalt when acted
upon by the humus-acids of the soil or exposed to the
alternations of atmospheric conditions (as in the famous Kase-
Keller of Bertrich), or exposed to the alternate action of
sea-water and air in the parts contiguous to the shrinkage-
cracks of an eruptive dyke which intersects a coast-line,
excellent examples of which may be seen in the coast-sections
of Northumberland.

On the one hand there is often great difficulty in drawing
the line between granite and gneiss ; and on the other there
is even greater difficulty in making a distinction between the
schists and mere phyllites. This difficulty is increased by
the frequent appearance of ' sheen surfaces ' (to which Prof.
Bonney has lately drawn attention), so commonly met with
for example on the original planes of lamination of the meta-
taxically-altered argillaceous shales of North Devon and the
Rhine country. This sheen surface is different from the
phenomenon of 'slickenside ' a mere film on the surface of
the laminae and may be perhaps referred to the freer access of
water to the original lamination-planes than to the interior of
the laminae. The proposal of Prof. Geikie (Text-Book of
Geology, p. 121) to restrict the term ' argillaceous schists ' to
these is worthy of attention ; yet we feel some dissatis-
faction in applying the term schist to these rocks at all.
Moreover it may be, I believe, observed in some of the oldest

OF THE

UNIVERSITY

GENERAL BEMABKSKN 89

Alpine schists as an accessory and subsequently-induced
character.* Even greater difficulty seems to exist in applying
the term schist to crunched and baked masses of altered
sandstone with highly ' slickensided ' vitrified surfaces, such as
is included in the intrusive mass in one place in Charnwood
Forest ; f similar examples of which on a smaller scale I have
observed as included fragments along the junction of the
granite with the sandstones of Cretaceous age in Saxon
Switzerland. Again in the Alps, in the higher Swiss valleys
and the deep gorges cut down through the Triassic limestones
of the Semniering Alps in the extreme east, one comes across
great masses of rock often intersected by veins of secondary
quartz and containing within them, scattered more or less
rolled fragments of vein quartz, around which a pronounced
flexure has been given to the cleavage (for I do not from
macroscopic observation think it is anything more), as if this
were the result of considerable shearing accompanying the
pressure which has given to the rock its present structure.
Blocks of such rock may be seen frequently used as road -posts
in the Alps, and in the Inn Valley I have seen them shaped
into millstones. My own impression is that they are nothing
more than palaeozoic ' scree '-materials subsequently altered
by pressure crushing and shearing, and that even to these the
term ' schist ' would be in a strict sense inapplicable ; they are
in reality slates with very irregular cleavage.

By far the most extensive exposure of such rocks in the Alps is the older
Verrucano and Anlhracitformation (HeimJ) of the Todi-Windgallen Group,
which that author has worked out so thoroughly. It is in these half-
crystalline representatives ('Vertreter,' Heim. loc. cit.) of the palaeozoic
formations in the Central Alps that we find perhaps the nearest approach to a
true schist; yet even here we should probably err in ascribing their crystalline
character so far as it goes to the excessive pressure to which they have
been subjected. The crystalline minerals of which they are largely made up,
are probably only the finer detritus corresponding to the coarser conglomeratic
masses intermingled with them (containing blocks of gneiss, granite, porphyry
and red jasper), the whole being probably derived from an archaean Alpine
region, as this was subjected in the ea-liest stages of its elevation to the
scouring and disintegrating action of the earlier palaeozoic tides which rolled
over it ; so that at the time of its first emergence as dry land, it had
undergone considerable marine abrasion, the removal of the materials of the
later archaean rocks from the crest of the region of elevation furnishing the
detrital material of the unfossiliferous 'transi ional Grauwackenformation,'
while its removal from the region of elevation facilitated the extrusion of the
yet imperfectly consolidated Central-gneiss series and the developement of the
'fan-structure.' Subsequent hydro-chemical action would account for the
secondary minerals (talk, sericite, chlorite) which occur in them ; so that when

* Compare Bonney, Pres. Address, 1886, also App. ii, note H.
f 1 refer to the rock quarried in Bazil (or Brazil) Wood, which I know very
well. It may have been once a rather gritty slate.
Mech. der Gebirysbildung. Bd. i., pp. 4152.

90 BOCK-METAMOBPHISM.

subjected to the enormous pressure and shearing of later mountain -building
movements, they would become sufficiently welded together to simulate in
some instances the character of a schist or gneiss, some of the rocks even
approaching (macrosco, ically) the character of gneiss in which mica is
replaced by anthracite. Heim remark* (op. cit. p. 42), "While the Verrucano
and rocks of the Casanna-schist Zone are often in hand -specimens [macro-
scopically] undistinguishable, they differ essentially as a series in that the
Verrucano shows no tendency to pass over into amphibolites and massive
gneisses or amphibolite-gneisses, as do the rocks of the Casanna Zone." A
detailed microscopic examination is, as he remarks (op. cit. p. 43), much to be
wished. What the result of such an investigation may be would seem to be
foreshadowed by Bonney's microscopic researches in the less extensive
representatives (Val Orsina conglomerate with its associated grits and slates)
of Heim's Verrucano in the Western Alps,* in which he finds no trace of the
characteristic foliation and 'metamorphism' of a true gneiss or mica schist.
The history of the Central Alpinet Verrucano, which I have suggested above
as to me the most probable, applies I believe more or less to the Granulitic
Series of Lehmann in Saxony (including the Obermittweida Conglomerates) ;
to the rocks (perhaps) of the Sparagmite'tage of the neighbourhood of Bergen
in Norway (with which Heim compares the older Verrucano) ; and to the
Huronian Conglomerate near Sudbury in Canada,J in which we have perhaps
the latest records yet deciphered of rock-building in the pre-Cambrian stage
of the history of the earth's lithosphere. It is only by piecing together such
fragmentary records from different parts of the world that we can hope ever
to arrive at anything like a continuous and consistent view of the gradual
changes of physical conditions and their geological operation, which shall
bridge over for us the vast gulf that appears to exist between the palaeozoic
and archaean rocks. We have no more right to assume the existence
originally of an universal gulf between the Cambrian and Pre-Cambrian
rocks, than between the palaeozoic and neozoic series: though great gaps
undoubtedly occur, not however in every region marking the same period of
the history of the lithosphe'-e ; arid if Prof. R. D. Irving's paper on the
Huronian Group had done nothing else, it would at least be of the greatest
value in bringing this idea more prominently into view. Such cases as are
here cited will serve, it is to be hoped, as the foundations of some of the piers
upon which the labours of the geologists of the near future may be able to
bridge over the vast abyss. This can however only be done as the scientific
geologist frees himself from all empirical theories all fanciful ideas of
'regional metamorphism,' which are so dear to the (orthodox) geologist, and
approaches the subject with a clear and open mind (enlightened by a first-
hand acquaintance with physical and chemical laws), prepared to accept the

* "A supposed case of metamorphism of an Alpine rock of Carboniferous
age." (Geol. May., dec. ii, Vol. x., pp. 507-511, 1883.) The more recent
published description by Dr. Grubenmann of the Airolo series can scarcely be
said to have proved very much. (See App. ii, Note T.)

t In the Eastern Alps the Austrian geologists have worked out a more
distinctly differentiated palaeozoic series, the 'Silurian' (Von Hauer ' Die
Gcoloijie^ p. 244), having been traced in a continuous zone ('fortlaufender
Zng') from Kitzbiihel in Tirol to Gloggnitz on the south side of the Vienna
basin, the rocks consisting mainly of Thonschiefer, Grauwackensandsteine
and dolomitische Kalksteine.

Bonney, Q.J.G.S., February, 1888.

The actual transition from the Archaean to the Cambrian exists no doubt
beneath the floors of the permanent ocean-basins, the original areas of
depression in the contraction of the lithosphere, accentuated by the weight of
the water which flowed into them, as aqueous condensation proceeded part
passu with the cooling of the lithosphere. (See Heim and Suess, op. cit.)

GENERAL REMARKS . 91

teaching of the evidence (macro- and microscopic) which Nature alone can
furnish. The jecord is undoubtedly but fragmentary; yet the writings of
such historians as Mommsen show us how much can be learned in the
department of human history when scanty fragments are arranged in their
natural order. Bonney's later papers* are simply invaluable as pointing in
this direction. His descriptions of the facts are so clear, and his conclusions
so logically and tersely stated, that they do not admit of being summarized :
they must be read and read ngain by the earnest student.

Nowhere is the archaean succession better worked out than it has been by
the officers of the Saxon Survey in the Erz-gebirge (Credner, op. cit. 6th ed.,
p. 387, fig. 131). There we trace the record of the gradual transition from (i)
the Ur-yneiss (stage for the most part of dry fusion), through (ii) the
Glimmer srhiefer, (stage of incipient aqueous action under great pressure and
at high temperature, perhaps all the water in contact with the lithosphere
being either in the condition of super-heated steam or in the liquid -gaseous
'critical -state') to (iii) the Pht/llttformation, (stage probably of extensive
liquefaction of water still highly superheated). As dissipation of energy
went on. we should have in some regions the later Cambrian conditions
anticipated to some extent [and of course the converse may be true that in
some regions the conditions which dominated the later Urschiefer-stage may
have lingered on in the Cambrian] ; giving us a gradual ascending transition
from the highly micaceous rocks (noch mit starkem Glimmerglanz) into the
slates (Thonschiefer) above, which in Thlirineen, in Vogtland, in the
Erzgebirge and in the Fichtelgebirge were first described as Unter-Kambrium
(as the Huronian was in America). See Credner, op. cit. p. 400. Compare
also Dr. Barrois' ' Observations preliminaires sur les roches des environs de
Lanmeur' ; Ann. de la Soc. Gel. du. Nord., T. xv, p. 238, et. seq.

There is some warrant for the supposition that the Cambrian marks the first
stage of full oceanic conditions on the globe, the archsean phyllites (Ur-
thonschiefer) being perhaps the products of sedimentation in the earlier stages
of aqueous condensation, which must have marked the setting in of the fifth
of Zollner's phases of the existence of worlds, f

On the supposition that the crust had now sufficiently cooled to allow of a
general condensation of water upon it, the vast accumulations of the materials
of the Cambrian slates, grits and conglomerates can be understood as
resulting from the destruction of, and deposition as sedimentary detritus from,
the cooled slaggy crust and its volcanic ejectamenta. by the great tidal waves
which swept over and levelled down the inequalities of that crust, even
though (as some have thought) there may have been no very general elevation
of dry land ^,bove the ocean- waters in the Cambrian and Silurian periods.
And if this were so, it helps us to understand the widespread existence of
crystalline allothigenous constituents (mica, &c. ) in the Cumbrian and Silurian
slates. Such a hypothesis seems to account for the observed unconformity
between the Archaean and Cambrian rocks, a fact, the significance of which
seems to be heightened by the abandonment of the notion of the Cambrian
age of the Huronian, and the relegation of it to the Archaean.

* See especially his three papers in the Q.J.G.S., February, 1888.

t Briefly these may be stated thus:

1st. The nebulous phase (condit on of glowing gas).

2nd. The glowing liquid phase (as iu fixed stars with constant brilliancy).

3rd. Developement of a solid glowing crust as in fixed stars of variable tirilliancy.

4th. Bursting of the thin crust, and extensive eruption of the glowing fluid interior.

5th. Gradual thickening of the solid crust and condensation of water upon it.

The meteoric theory ' of Prof. Brauns of Halle, ('cette audacieuse liypothese,') may be
passed by here. It has rece ; ved the treatment it merits at the hands of Prof, de Lapparent;
see La Formation de I'Ecorce Terrestre (pp. lu-14) ; Brussels 1888. It has certainly furnished
an exercise for the mathematic.an, but it must be recollected that the out-put of the
mathematical mill depends upon what is put into it ; and the crux of the whole question is
as to the nature of this.

92 BOCK-METAMORPHISM.

Considerable confusion has also grown up in the use of
the term ' slate,' shales and slates being not infrequently
confounded. A rock may of course furnish a very good
' roofing-slate ' in the workman's sense of the term (as in the
case of the Collyweston and Stonesfield ' slates ') and not be a
' slate ' at all according to the petrographical use of the
word. Where the pseudo-cleavage is nothing more than the
accentuation by pressure of the original lamination of a shale,
the rock is still a shale and nothing more, even though its
argillaceous composition is as perfect as that of a true clay-
slate, and secondary minerals have been developed on the
lamination-planes as separation-products, by long-continued
action of percolating water. Whatever the condition in which
such a shale is now found, it would be designated Schieferthon,
in German nomenclature ; and thus a clear distinction is
drawn between an indurated and slightly altered shale (in the
sense just indicated) and clay-slate or Thonschiefer.

How far can we attach any meaning to the term 'metamorphism,' as applied
to the distinctive morphological characters of the archcean rocks?
Passing in review the main points of the evidence, we note :

1. The alleged evidence of the archsean existence of low forms of animal-
life. Any one, who, with a fair command of the German language and a
moderate acquaintance with Protozoan forms, will form a judgment of the
merits of the Eozaon-Oontroversy from a study at first-hand of Mobius'
splendid Monograph, "Der Bau des Eozoon Canadense" (Palceontographica,
Cassel, 1878). and not from versions thereof which have appeared from time
to time in the English language, must admit that the evidence of the organic
origin of that structure is very slender indeed. And in the face of the
overwhelming physical evidence against the possibility in archsean time of life,
as we know it, on this globe, it is not too much perhaps to say that the
evidence in favour of Eozoon representing a living form must be made
infinitely stronger than it has yet been made in order to establish the case.

For my part I fail to see that there is any better evidence of the organic
origin of the so-called Eozoon structure than of such mineral infillings as may
be seen in a section of a slab of 'Gotham marble' or in such as those figured
in Lehmann's Altkryst. Sehieferf/est, Tafel xvi. fig. 6. which are described by
that author in a note as "a layer (in a granulitic rork) in which amphibol has
grown inwards from the surface." Compare also Dr. Barrois, op. cit. T. xv.,
pp. 241, 242.

2. As to the existence of vegetable life, graphite being accounted for (App.
i, Note I) as the result of the action of known chemical and physical laws, its
presence can no longer be adduced as evidence of the existence of archaean
vegetation; nor does 'hydro-carbonaceous' matter in the shales of later pre-
Cambrian time necessarily imply the existence of organic matters.*

3. Iron-ore (both F 2 O 3 and FsC^). Though the former (hydrated) is well
known to be precipitated by oxidation of iron-salts in solution in waters
cont lining organic acids, it can equally well be produced by the direct
combustion of iron- vapour, while Fe 3 O4 is as is also perfectly well known in

* See also my paper ' Further Notes on Dissociation by Contact-Action,'
Chem. News, No. 1505 ; also that on 'The origin of Graphite in the Archaean
Rocks,' B. Assoc. Report, 1888. Compare App. ii, Notes P,T,V.

GENERAL REMARKS. 93

the chemical laboratory the direct product of the reducing action of iron at a
red heat on superheated steam. The oxides of iron are therefore put out of
court as evidence of the existence of archaean vegetation ; aud with them
must go also the iron-carbonates of the Huronian.

4. Crystalline Limestones, as has been shown, could have been formed
abundantly (both ^y dry and wet reactions) by ordinary known and demonstrable
chemical changes under such gradual changes of thermal conditions as are
deducible from the principle of dissipation of energy, and may have been for
the most part originally crystalline, and are not necessarily metamorphic.

5. Quartzites (including the finer Grauwacke) being seen to represent some-
times simply the excess of &iO 2 which separated out from such reactions as have
been considered, and especially by COa in solution where (and when) water
was locally condensed on the then-formed crust, there is no necessity for
regarding them as altered 'clean sandstones,' or in any sense as per se the
results of later metamorphic action.*

6 True Foliation appears to be quite exceptional in the earlier palaeozoic rock
(leaving out of account of course cases of contact-metamorphism), and where
it does occur in sedimentary rocks of undoubted post-archaean age, seems to
amount to no more than 'cleavage-foliation,' producing in such cases mere
phyllites, which are common in the later pre- Cambrian rocks.

Are we not justified in saying that the extent of subsequent metamorphism
which the archaean rocks have undergone on anything like a general or
regional scale is for the most part metataxic ; the fact of their having under-
gone any extensive paramorphic change, beyond such local secondary paramor-
phism ' as later igneous rocks of local origin have undergone, being unproved ?

From what has been advanced with reference to (a) the possible (solid-
liquid) 'critical state' induced under great pressure on certain minerals, (b)
the fluxing-action of traces of accidental mixtures, (c) the thermal energy
developed by friction where the local mineral composition was such as to
lead to crushing rather than incipient fusion, it would be absurd to deny
that slight changes of mineral composition (secondary paramorphism) may
have occurred in some cases as the result of mere mechanical action pari
passu with metataxis and metatiopy. When however we realize the
universality of the resultant operation of the combined factors of time^ and
ordinary chemical atomic change, are we not justified in saying that the onus
probandi rests with those who would attribute the whole, observed result (in any
case) to pressure, and that the fallacy post hoc frgo propter hoc runs through
much that has been asserted and written on the subject of 'dynamic or
pressure metamorphism ?'

Eight truly has Kalkowsky remarked (Lithologie, p. 56) : " Bisher ist der
'hohe Druck' in der Lithologie wie in der Geologic noch vielfach ein dens ex
machina, der auftritt, wenn andere Erklarungsversuche nicht auszureichen
scheinen ; " which is as much as to say in plain English that pressure-
metamorphism is a 'refuge for the destitute.'

* In what has been urged in this work as to the chemical origin of the
salinity of oceanic waters, and of the primitive quartzites and unfossiliferous
limestones, I find I have quite independently and unwittingly trodden in the
footsteps of Macfarlane ('Origin of the Eruptive and Primary Kocks,' pp.
71-73), and of Dr. Sterry Hunt, (Canadian Naturalist, vol. vii, p. 202, quoted
by Macfarlane, loc. cit.)

t The writings of Suess and Heim, so far as they deal with the history of mountain sys-
tems as details of the larger subject, the history of continents, are of the highest degree of
importance from this point of view.

94 , ROCK-METAMOBPHISM.

CONCLUSIONS.

If now we review the facts and arguments contained in the
foregoing dissertation, and take into account those given in
the appendices, several important conclusions would appear to
be suggested.

1. It is a vain and useless task to think of finding any
one principle of metamorpliism, since this is exhibited in
Nature in various degrees and in various phases. In the
higher and more complex phases of change all the four
principles discussed in this work are exhibited as having come
into play, not necessarily simultaneously, but in an order of
succession determined by general laws of nature ; in other
words the laws of nature being (so far as we have any
possibility of knowing) persisbent, unchanged, and unchange-
able, admit however of an indefinite variation in the proportions
in which their operation is manifested in any particular field
of action. The almost infinite variety of their collocations is
not therefore so much a qualitative as a quantitative variation;
and to these the variations in the phenomenal results must
correspond. We have in fact all gradations of change, from a
simple metataxic change in the cleavage of slate, or a simple
metatropic change in the crystallization of a limestone into
a marble after fusion under pressure, to the most complex
changes observable in regions of contact-metamorphism.

2. When we come to consider what is commonly under-
stood as ' regional metamorphism,' very large elements of
doubt and uncertainty are introduced; and considerable
non-provable assumptions have to be made in order to con-
struct anything like a coherent theory. Some of the most
important data upon which it has been attempted in some
quarters to build up such a theory are found to admit of a
different explanation from that which has been given to them
by the advocates of the theory; and the relative values of
these different explanations is not a matter of opinion, but a
question of agreement or disagreement with known laws of
nature.

3. The important revelations which the spectroscope (as a
supplementary instrument to the telescope) has made in the
last two or three decades, taken together with a firm grasp
of the principle of dissipation of energy* furnish data which
cannot fail to throw light upon the history of the developement

* It is because this great principle is ignored, or at least but feebly grasped,
that some geologists follow so blindly the rigid ' uniformitarianism ' of which
we hear so much. Until geologists of that school cotne to realize the vast
importance of this one simple physical principle, we cannot admit that their
creed is a ' rational ' one .

CONCLUSIONS. 95

of our Earth ; the only assumptions being (1) that the same
universal laws have operated in that deveiopement as are seen
in operation in other bodies which roll through space ;* (2)
that the mass of the earth including its atmosphere has
remained the same from the beginning, its material having
neither diminished nor (with the slight exception of added
meteoric matter) increased. Deductions from the known
principles of thermal chemistry and physics lead to the
conclusion that in the history of its deveiopement the Earth
must have passed through a pre-oceanic stage, in which
deposition of the more stable minerals occurred to form a
non-consolidated crust in the presence at high temperature
and pressure of that chemical body whose composition is
represented by the formula H 2 ; and in the minerals which
for the most part constitute the archaean and pre-archaean
rocks we can recognise just those which, from their known
high degree of stability, we must regard as most likely to
have been formed at such a stage of the Earth's deveiopement.
This leads to the further conclusion that the process by which
the archaean gneisses and schists were formed (so far as their
essential mineral-characters are concerned) was essentially
1 diagenetic ' rather than ' inetamorphic.' If this be admitted,
such phrases as " the highly-metamorphosed archaean gneisses
and schists " must be relegated to an obsolete nomenclature of
geologic science.

4. The archaean rocks themselves, wherever we have an
opportunity of studying them in anything like their full
deveiopement, exhibit a general, though not uniform, pro-
gressive series of changes of mineral character, from the oldest
mixtures for the most part of the most (thermally) stable
mineral compounds, quartz and felspar (the granitoid funda-
mental rocks), through well-foliated gneiss (with its many
varieties) into the schists and phyllites, in which the silicates
themselves tell the tale of a gradual increase in the two main

* Pfaff (Geol. als ex. Wiss., p. 31j thus summed up the evidence of this,
fifteen years ago :

1. " No physical or chemical law, and in particular no known phenomenon
can be brought forward, which would oppose the view of an original glowing
(molten) condition of our Earth.

2. As a necessary consequence the oblate form of the Earth and the
increase inwards of temperature, which we in fact observe, came about.

3. That we can recognise in the Sun and in other of the larger heavenly
bodies the same glowing'condition, makes it the more probable to us that the
Earth was once iu the same condition. The latter moreover, as one of the
very small masses as compared with the Sun, must ha e cooled, so that its
crust solidified, so much sooner than this can happen to such an enormous
mass as the Sun."

96 BOCK-METAMOBPHISM.

factors which have determined their differentiation, (a) water,*
(b) heavier (but less chemically-active) basic oxides, the high
temperature of what we may call the ' Ur-gneiss stage ' having
been upon the whole too high for the formation of the silicates
of the latter on anything like a general scale, and causing
their deposition mainly as oxides, sulphides, and fluorides
scattered as accessory minerals through the Ur-gneiss (in its
many modifications) or precipitated more copiously in great
intercalated metalliferous zones (' Fahlbander ') often miles in
area. And the truth of this as a deduction is not affected,
though the difficulty of reading the record of it is increased,
by the vast disturbances (resulting in fracture, faulting, over-
thrusting, and overfolding) which have since interfered with
the original order, and the enormous waste they have suffered
in furnishing materials for sedimentary rocks, the proportion
of SiO 2 (either free or in combination) in the one series and in
the other being much greater than that of all their bases
taken together.t

5. We thus come to regard the archaean series of rocks as
representing upon the whole the primordial (first-formed)
earth's crust, from which the siliceous materials of the
sedimentary rocks have been for the most part derived : the

* This water was still for the most part highly superheated, in a condition, that
is to say, such as that in which it operates in the high-temperature stage
of contact-metamorphism in converting a clay-slate into a crystalline schist in
immediate contact with granite, &c. Lawson's description of the crystalline
schists in his Essay (op. cit.) may be referred to in illustration of this.

t Of all the chemical elements Carbon alone can be mentioned (after
perhaps Hydrogen) as endowed with such marvellous and varied potential ties
as Silicon. Whether we note their tetravalency as elements, their remarkable
relative positions in the Periodic or Natural Arrangement of the Elements (as
broached by Newlands, and since worked out to a greater developement by
Mendeljeff, Reynolds, Carnelly, and others), or the part they severally play in
the economy of nature, we can hardly escape recognising a sort of conjugate
relationship between them. The chemical potentialities of Silicon being
called out mainly at high temperatures, and those of Carbon at more moderate
temperatures, they seem to stand, as it were, at the two opposite poles of
matter, dividing the empire between them into what we commonly call the
Organic and the Inorganic, but with very undefined boundary lines, along
which dwell a series of restless and turbulent tribes, the individuals of which
own no permanent allegiance to either, passing from the domain of each into
the other in the most facile manner. Is it too much to hope that the day will
come when the study of the chemical potentialities of the element Silicon
shall become the focus from which new light shall irradiate the chaos of
Mineral Chemistry, as the last decade or two have seen the confused
accumulation of facts in the 'Chemistry of the Carbon Compounds' brought
to a great degree into order and symmetry by the recognition of the simple
principle, that, "in the cliemical properties of Carbon alone lies the essential
fact, which determines the peculiar properties the Carbon-Compounds possess,
as compared with all others." ? (Wislicenus, Organ. Chem., 1.)

CONCLUSIONS. 97

fixation of C0 2 and oxides of some other non-metallic elements*
as also possibly of the halogens, to form carbonates, sulphates,
and haloid-salts, having continued on possibly into early
palaeozic times. The archaean stage of the earth's history is
thus seen to fall into a place in a natural order of develope-
meut, and one more chapter is added to the history of the
operation of that great Law of Evolution which is written
upon all created things.

^ 6. As the mists and clouds thus disperse our intellectual
vision begins to descry a boundary to geologic time, and the
physical geologist begins to feel that over this question he can
join hands with the astronomer and the natural philosopher.

* While much of the sulphur once existing in the elementary state in the
primordial atmosphere may have combined directly with metallic vapours to
form sulphides (the combustion of metals in sulphur- vapour being a fact well
known in the laboratory),* a large proportion of it no doubt underwent
combustion into S0 2 with the further formation of free sulphuric acid ; and
this free acid no doubt played a part subordinate to that played by C0 2 in the
decomposition of the earlier-formed silicates, the SiO 2 thus set free contributing
to the production of the grauwacke quartzites. The insoluble sulphates of
the alkaline earths thus formed were deposited, while the soluble sulphates of
other bases were removed in solution. In some cases the sulphates may
have been reduced to sulphides, which, as ores of the heavier metals (iron, zinc,
copper, lead, &c.,) are of such common occurrence

Appendix i.

Notes of Laboratory- Work,
a. Sulphur,

Sulphur was obtained in the 'vitreous state ' in two ways: (1) by pouring
molten sulphur into cold water ; (2) by just melting lumps of brimstone in
an open evaporating-dish, allowing a crust to form in the first stage of cool-
ing, perforating this and pouring out the portion still liquid. Some of the
residual 'crystallites' (App. ii. Note F.) were examined microscopically. In
some instances minute triangular thin plates of clear glass were found attached
to the edges of the crystallites, giving them a very serrated appearance.
These, when detached and examined microscopically, gave beautiful and
interesting results.

At the ends of these crystallization sets in rapidly so as to point off the
lath-shaped bands with devitrified wedge-shaped interstices. Gradually,
but not uniformly, devitrification is seen setting in along the margins of
contiguous bands of the glassy material, and here and there the crystallization
cuts transversely into the glassy prisms, forming little nests of crystals,
which, in a few minutes (in cases where they begin to form at opposite points
of the prism) coalesce and completely intersect the glassy band.

As devitrification proceeds in its earlier stages a marked pleochroism is
exhibited along the devitrified lines, while the bands retain in the interior
their isotropic character as glass. In the course of half-an-hour with strong
sun-light devitrification may be so far advanced as to render the sulphur
opaque. In one very thin plate of sulphur-glass, upon which the full-powered
sunbeam was allowed to fall directly as well as by reflection from the mirror
of the microscope, I was able to watch the progress of the devitrification ;
and in that case the plate became opaque in five minutes. Sunlight is thus
seen to aid in devitrification of sulphur as well as of phosphorus.

Latent heat of vitreous sulphur.

1. Vitreous (plastic) sulphur obtained in the usual way by pouring sulphur
at about 400C into a deep jar of cold water.

2. One portion of this on being hammered on a cold stone slab became in a
minute or two so hot that it could not be comfortably held in the hand.
The brown colour gave place to a pale yellow, partly stringy, mass very tough.

3. The portion treated as in (2) was placed out of doors on a cold stone
slab until it was cooled down so far below the temp, of the room as to show
considerable cold (40 to 50 on the scale) when placed on the metallic face of
the thermopile ; yet after a few minutes exposure to the air of the room it
gave a large reading for heat on the scale (100 to 200) when again placed on
the thermopile. For this purpose it was lightly pressed down on the face of
the pile with a glass rod so as to get a large surface of contact. The mass
hardened and stiffened rapidly.

4. A piece of the same hammered mass was examined after between one
and two hours under the microscope in reflected light (it had then become
quite opaque) and was found quite crystallized, with the exception of a few
small granules of brown amorphous sulphur.

5. After about 18 hours the residue of the hammered mass had become quite
hard, inflexible, and brittle, falling easily to a crystalline powder when
broken.

APPENDIX 1. 99

6. Experiments with unhammered soft brown portions of (1) after con-
siderable exposure (1 to 2 hours) to the air of the room gave slight heat
indications (about 10) when laid on the pile ; and 40 to 50 when lightly
pressed upon it to secure a large surface of contact.

7. Another portion of the fresh soft vitreous sulphur (1) was placed under
a pressure of about 2 cwt. between two hard white porcelain tiles all night.
In the morning it was about half devitrified, yellow and crystalline in those
parts where the pressure was heaviest. When the flat plate of S thus procured
was placed on the Th. pile (after some time of exposure to the air of the
room) it gave a reading for heat of about 40, and when gently pressed on the
face of the Th. pile nearly 200. Devitrification went on so rapidly that after 3
or 4 hours it gave only 10 when placed on the pile ; and on examination
under the microscope it was found to be almost completely crystalline. Next
morning the plate was quite brittle.

8. Portions of the vitreous sulphur (1) allowed to crystallize under ordinary
physical conditions were not completely devitrified before the end of 3 to 4
days.

9. A thin slice of the vitreous S. (1) when fresh was found to be quite
isotropic in polarized light, but the crystalline structure developed itself
rapidly from the edges inwards, and the next morning it was found quite
opaque and devitrified.

The facts just described clearly point to the presence in the cases described
of latent heat of vitrification, this heat escaping as devitrification progresses.

The specific heat of sulphur being '118, we can calculate from the experi-
mental results the latent heat of vitreous sulphur in the form of klinorhombic
prisms as 2'27, since the rise of temperature which this form undergoes on
sudden transformation into the ortho-rhombic form=:12 'l. (Wislicenus, op.
cit., 236).

Again, the rise of temperature given (on the same authority) on page 87 of
this work, enables us to calculate the latent heat of the plastic form of sulphur)
thus :

188 (specific heat of sulphur) x (110 - 93) = 3'196.
Whence also latent heat of

b. Phosphorus.

1. A portion of amorphous P. and a portion of vitreous P. were placed
under a pressure of two cwt. between white porcelain tiles.

After about 18 hours the crystallization under pressure was very marked in
both, particularly in the specimen of red P. ; this was quite hem i- crystalline
under the microscope, between two plates of glass, but showed no trace of
pleochroism.

The compressed plate of vitreous P. exhibited a great variety of phenomena.
A large part of its area was occupied by a vitreous ' basis ' through which
were scattered innumerable minute aggregates of crystalloids. There were
also large patches of opaque amorphous P. A few faintly developed ' belonites '
and ' globulites ' were seen.

As moisture and 62 gradually found their way in between the glass plates,
crystalline bars and crosses were developed. These were brilliantly iridescent
in polarized light, and especially beautiful between crossed nicols, when the
ground around them was dark. The lines of colour on these rods indicated the
prismatic form of the crystals, which must be regarded, I think, as H 3 P04,
free from hygroscopic moisture.

2. Examined specimens of (a) red P., (b) vitreous P., both of which had
been under pressure of nearly two cwt. for 6 days and nights continuously.
Pressed plate removed and placed at once between glass plates which had
been previously washed (1) with fuming HNO 3) (2) NH 4 HO, (3) distilled
water, then well dried,

100 KOCK-METAMORPHISM.

In the red specimen (a ) the same effect is seen as in the specimen before
examined ; but the crystalline portions transparent between crossed-nicols
are larger and more definite in outline. Here and there bunches and prisms
of well-formed crystals are seen.

In the vitreous specimen (b) a great variety of things were observed.
Devitrification could be observed even macroscopically. Bunches and masses
of red amorphous P appearing, some of which are soon seen to develope (in
diffused light) crystalloids which a little later began to group themselves into
crude cry stall ographic outlines.

Trichites, and microliths (straight) with obtuse ends were observed. Both
classes of bodies with a very definite outline. Some of them quite pellucid
(with vitreous interior) others semi-opaque (containing amorphous P. ) Masses
and patches of true crystalline P. of the regular system, and ' crystalloids '
(Vogelsang) Zirkel ( Mic. Bes. der. Min. u. Gest.) Globulites (with definite
outline) some pellucid others semi-opaque. All these forms (except the
crystals) undergo slow change with exposure to light.

After one or two hours a periphery of glassy HPOs was formed as a result
of oxidation, and inside this (though not in every case close to it) a
phenomenon of surpassing beauty was observed. In a transparent film of
liquid matter a constant and rapid circulatory movement was seen which
one could not help comparing with the circulatory movement of the protoplasm
in the living cells of plants. This was at first very puzzling : but on further
watching and reflecting upon the phenomenon, I saw my way to a probable
explanation of it. A glassy mass was seen in the middle of the larger of
these little areas, which was nearly opaque between crossed-nicols and
produced no colour-effect upon sunlight transmitted through a nicol rotated
below it. This I take to have been HPOs (glassy metaphosphoric acid) ; and
the circulation in the liquid-film therefore but a manifestation of the molecule-
forming energy which was being there displayed. The minute currents in the
film were probably produced by the local generation of heat as molecules of
H 3 P04 were successively built up. Five or six hours later the vitreous mass
(HPOg) was much diminished and had parted into two small masses. Next
morning the whole area had solidified and acquired the optical characters of
crystalline H 3 P04.

c. Silica.

1. A specimen, of glassy silica precipitated from Na 2 Si0 3 as Si0 2 + xH 2
several years before in my laboratory.

Portions found quite devitrified and opaque : too thick for examination by
transmitted light. In strong reflected sunlight the whole surface was found to
be crudely crystalline, by bringing successive elevations of it into the field of
vision. Here and there, where the light pierced the thinner portions, the play
of colours when the lower nicol is rotated showed the crystalline texture.
Crystal-growth at edges very distinct ; also in thin transparent patches of
interior. With crossed nicols it allowed white light to pass freely.

2. The fine impalpable powder precipitated several years ago in the same way
as before was examined in transmitted light, also the fine powder obtained by
the desiccation in the air of the laboratory of the hydrabed filtered-off
gelatinous precipitate obtained by passing silicon tetrafluoride through water,
by the well-known reaction

3 SiF 4 + 4 H 2 O = 2 H 2 SiF G + Si(OH) 1 .

Definite crystalline form detected only in a few particles. The particles
generally appeared to be sub -crystalline masses of partly devitrified glass,
producing a certain play of colours when rotated in polarized light, and
giving free passage to white-light when viewed between crossed nicols. In a
f*w of these fine particles pellucid globulites were well seen with a J-inch

APPENDIX i. 101

objective. Generally the particles are distinctly angular, some of them more
elongated with pyramidal ends, the angle of the pyramid being that of rock-
crystal, and in some instances minute pyramidal crystals were seen as a
growth on the edges of the larger masses. In one case the minute crystallites
were seen (-inch Ob.) grouping themselves into a crude crystalloid of the
normal shape of rock-crystal pyramidal at each end.

3. In a specimen of glassy silica only two or three months old beautiful
moss-like growths of crystalloids were seen in the translucent margins of the
thin edges of a partially devitrified mass. The colour-play when a rotated
beam of polarized light was passed through them, and their extinction between
crossed nicols established their crystalloid character. (Zirkel).

4. Examined under the Mic. specimens of hydrated 8162 powder which
had been precipitated about three months.

The minute powder was made tip of angular particles of devitrified SiOa as
in the older specimens examined before. A few larger plates were obtained
from this powder, strongly iridescent. Under the M ( + Nic) a splendid
spherulitic structure was observed ; very marked and very beautiful : some of
the discs well rounded, others with a wavy periphery. The plates were too
thick to give well-defined outlines of these spherulites, with powers higher
than 1-inch obj ; but the play of colours when a Nic. was rotated below in
bright sunlight, seemed to indicate some incipient crystallization.

It was impossible to avoid being struck with a certain resemblance of this
spherulitic structure to that observed in some volcanic glasses.

5. A portion of clear solid glass of hydrated silica, which had been
precipitated about three months (A), and a portion of more devitrified glass which
had been precipitated about three years (B) were gently dried and weighed,
then calcined in B-pipe furnace for over half an hour.
Results:

A was melted down to almost a clear glass at the bottom of the crucible
with perfect extinction between x nicols and was found to have lost 50'3 per
cent, of water by weight.

B remained as a porous pumiceous mass and was found to have lost 23 '9 per
cent, by weight of water.

Slow devitrification is therefore attended with loss of water in this case.

d. Solvent action of the Humus-acids.

Little is to be learned from the scientific literature of this country of these
acids, which consist of a series of carbon-compounds of no great stability.
As acetylene is a hydro -carbon product of incomplete combustion of lighter
hydro-carbons, and therefore in one sense a reduction-product from them,
by the partial combustion of their hydrogen into steam ; so in the process of
slow oxidation of organic matter these bodies result from incomplete oxidation
of the higher carbon compounds which are formed in the earlier stages
of decomposition in water of woody fibre and other vegetable tissues.
Ultimately they may be oxidized into CO 2 and H 2 0, which are the ultimate
products of combustion of the same non-nitrogenous substances. The
humus-acids are found to be capable of replacing CO 2 , and appear to be
intermediate in energy as acid constituents of salts between acetic and
carbonic acids. Much experience of waters containing them including
analysis of many samples has shown their potent solvent action on iron,
lead, and zinc among the metals. All this has been more fully treated of in
papers already published.* Here we are only concerned with the part they
play as solvents of minerals.

* See paper by the author in Proc. Itist. of Civil Engineers, vol. Ixxxv,
(1885-6).

102

BOCK-METAMOBPHISM.

Some time ago a favourable opportunity offered itself to the writer for
procuring a considerable quantity of these colloid extracts, as the result of
precipitation in an open storage reservoir, which received the overflow from a
spring issuing from the sands at the base of the Upper Bagshot. A year or
so previously a drain had been laid for some distance to this outflow a few
feet underground. The drain was constructed of open-jointed pipes ; and to
prevent the sand getting into the pipes a great quantity of heather-litter was
rammed in above them before the trench was filled up. The decay of this
underground charged the water with organic matter ; and, as this was only
incompletely oxidized in the storage- reservoir, the cemented bottom of the
tank became covered over in about six months with a slimy colloid mass,
containing some 50 per cent, of crenic acid freely soluble in alkalies. It
occurred to me that it would be interesting to ascertain experimentally the
effect of this upon some of the minerals which form the more common
constituents of many rocks. For this purpose two or three fragments of each
mineral were taken (all dust being carefully eschewed, in order to avoid errors
in weighing due to any accidental loss of it), well dried in an air-bath at
120C, then placed in separate flasks with 250cc. of distilled water in each, and
approximately equal quantities of the fresh colloid material taken from the
reservoir. The flasks were tightly corked, and allowed to stand at the
temperature of the laboratory as long as the water continued to give off a
perceptible odour, resembling that of ordinary pond-silt. This they continued
to do on being occasionally opened for about three months. After the odour
ceased, the organic acid being presumably destroyed by slow oxidation, the
minerals were taken out, dried again severally at 120C, and weighed. The
results are given in the following table :

Mineral or Rock.

Original
Weight dried
at 120 C.

Weight after
treatment
for 3 months.

Loss.

Percentage of
Loss.

Limestone (Hallstatt) .

gms.
4-280

gms.
4-265

gms.
015

350

Rusty iron 'pan,' part of a
conglomerated-gravel .

3-160

3-090

070

2-215

Chalk Marl . . , .

3-195

3-125

2-035

Muschelkalk .

3-070

3-060

010

326

Hornblende .

1-772

1.780

Augite

770

775

Orthoclase .

870

857

013

1-530

Sanidin (from the Trachyte
of Drachenfels) .

3-125

3-110

015

480

Bath Oolite ....

3-029

2-970

059

1-976

Oligoclase .

1-055

1-050

005

476

Obsidian

620

Mica (Muscovite) .

580

APPENDIX 1. 103

From these results it would appear that of the minerals experimented
upon ferric oxide yielded the most readily to the solvent ; next iu order
follow the more earthy varieties of limestone ; then the felspars and more
compact limestones ; while no solution at all of horneblende, augite, mica or
obsidian appears to have taken place. The very slight increase of weight in
hornblende and augite may be attributed probably to the oxidation in the
process of drying of the iron contained in them ; and this is borne out by
the fact that the augite crystal (from Bohemia), which was quite clean and
unweathered before the experiment, was found afterwards to present a
somewhat rusty appearance. In order to obviate the error which would

weighed portion

charcoal, by which means the access of atmospheric oxygen to it was
prevented.

The solvent action of organic (humus) acids on the oxides of iron pointed
out here, seems to throw light upon the occurrence of Carboniferous 'white trap,'
as described by Allport (Q. J.G.S., vol. xxx, pp. 538, 550, 556). The vegetable-
matter locked up in the coal-measures would furnish these and allied acids.
In addition to the researches of Berzelius, Boutigny, Mulder, Julien, and
others, reference may be made to the researches of Williamson (quoted by
Rammelsberg, Mineralchemie, p. 726) on the so-called Bogbutter from the peat
of Ireland, as establishing the veritable acid character of these bodies.

Allport (op. cit.) states also that "wherever this green rock (at Deepmore)
occurs, it invariably becomes lighter in colour as it approaches the coal or
shales, and near the junction is nearly white. In this state the constituents
are completely decomposed, but may still be recognised under the microscope,
the original forms being preserved. The iron appears to have been removed,
and the mineral is represented by pale grey pseudomorphs. Near the line of
junction the trap is always quite as much altered as the shales and coal, and
the alteration consists mainly in the decomposition of the ferruginoxis
silicates." (pp. 549,50.)

e. Devitrification of Flints, (cf. p. 46).

With reference to the devitrification of flints the following note may be of
some interest. Two specimens, in which marked changes had taken place by
prolonged exposure, were selected for microscopic examination.

Flint A. is so completely de vitrified as to have lost entirely all the character
of flint through an irregular layer about a third of an inch in average thickness.
It has become porous even macroscopically, and many of the pores are filled
with amorphous Fe 2 03.

Flinty band: (macroscopically compact) micro-crystalline ground-mass
between x nicols ; little specks of chalcedonic material scattered through it ;
some amorphous earthy-looking dust (probably, from the rusty colour of the
slide, Fe 2 O 3 ). On its outer surface this flinty band has assumed a porcellanic
character such as I have described elsewhere * as occurring on the exterior of
sarsens and fragments of millstone-grit, as the result of exposure.

Cherty band; (macroscopically stony and somewhat porcellanic, with quite
conspicuous perforations) ; ground-mass much more completely crystalline ;
perforations often lined with the deposited ferric oxide, which seems to have
been concentrated on their walls. The most striking feature however here is
the occurrence of secondary quartz in flint, highly luminous between x nicols.
It forms a sort of second wall behind the ferric oxide which generally lines, and
in some cases fills up, the perforations. There can scarcely be any doubt I
think from the relation which this 'mosaic-like' quartz is seen to bear to the

Pres. Geol. Assoc. Vol. viii, pp. 159-160.

104 BOCK-METAMOBPHISM.

ferric oxide, that the two are concomitant decomposition-products from solutions
of double azo-ferrous salts of silica and an organic acid. Special interest
attaches to the production of a 'mosaic-like' structure in quartz in the 'wet
way (i.e. through the intervention of water), so that such quartz has no
necessary connection with tridymite. The perforations are scattered with a
tolerable regularity through the mass, and are so approximately equi- distant
as to suggest the 'pores' or 'oscula' of a sponge. Whether this is the true
explanation or not of this interesting structure, we may safely assert that the
action of the solvent at a later period on this part of the flint has been much
greater than its action on the other part, in which the flinty texture has been
much less obliterated ; and we should not probably be far wrong in attributing
this difference to a difference of molecular structure in the deposited silica of
the original flint, owing perhaps to the more direct action of decomposing
sarcodic material in the one part than in the other.*

Flint. B. has a similar porcellanic character on the exterior to that described
already as occurring on the outer surface of A, but not quite so compact.
This flint is upon the whole less altered than A, but there appear to be in
places irregular masses of purer quartz than in the rest of the slide. The ferric
oxide has undergone concentration in a narrow zone just beneath the more
porcellanic outer layer. In the latter the ground-mass is distinctly micro-
crystalline and there is much opaque amorphous material which does not seem
to be wholly made up of ferric oxide. At two or three points are seen
structures with a striking resemblance to 'spherulites.'

In order to get some idea of the extent of the part which hydato-devitrification
plays in the degradation of flints, I have compared the loss of weight on
calcining, which three selected and fairly typical specimens were found to
suffer by heating to a glowing red heat for half an hour.

The following were the results :

LOSS.

No. 1. Specimen of Sarsen-stone from Nefcley Heath ) , QKO . .

(rather 'flinty') j

No. 2. Specimen of rusty flint which had undergone \ 1-674
a moderate amount of weathering. J

No. 3. Specimen of a bleached and chertified portion )

of flint. j d ' 125

The percentages are, as will be seen approximately in the ratio 1:2:4.
Hydato-devitrification seems therefore well exemplified in these changes. On
the other hand, a specimen of native black flint lost just 4% on being treated
in the same way. Loss by combustion of carbon, which amounts to about
06-'07% has been neglected.

At a recent meeting of the Geological Society (March 28, 1888) some
remarkable examples of corroded agates were shown by Prof. V. Ball in
illustration of a paper by him "On some Eroded Agate Pebbles from the
Soudan." My observations on the action of humus-acids on flints in the
Bagshot country led me to suggest that the humus-acids furnished by the
desert-scrub appeared to me from a consideration of all the facts of the case as
the most likely solvent ; for solvent action of some kind must have acted to
produce such features as the agate-specimens exhibited. I also stated that
Rammelsberg had pointed out that many minerals after being fused into a
glass are more readily acted on by solvents than in the crystalline state,
which, from considerations adduced, I regarded as due to difference of
molecular structure. Reference has already been given to this statement of
Rammelsberg's on page 45 of this work. I also stated on that occasion that
while the replacement at fusion-temperature of CO 2 in alkaline carbonates by

* The secondary quartz observed in this flint seems to me undistinguishable from that
observed in one of the altered "ash-beds" of Snowdou (see p. 59), and in some of Allport'a
specimens of devitrified pitchstones in the South Kensington Collection. (B, Mus., Nat.
His.)

t On the cementing (secondary) silica probably.

APPENDIX i. 105

Si O 2 was a matter of common laboratory-experience, it was not so easy to say
what the action might be in the ' wet way ' ; though it was impossible to deny
that Si 02 in some of its allotropic forms might replace C0 2 , and thus form
alkaline silicates which would be removed in solution. I have since
investigated this matter in my laboratory, and the result is an affirmative one.

1. A flint-nodule from the Chalk was split in two, and was found to be
entirely bleached and to have acquired the cherty character referred to above
to the depth of a quarter of an inch from the surface. Portions were chipped
off from this outer zone, and some of the cleanest selected. These were
pulverized in a mortar and boiled in a saturated solution of Na 2 CO3 for some
time (one to two hours). On filtering and nearly neutralizing the filtrate with
HC1, a precipitate of gelatinous silica was formed in one part of the filtrate
which was allowed to stand for a day or two, and more quickly in another
portion which was gently evaporated down on a water-bath at about 60C.

2. A fragment of fresh unaltered native black flint from the Chalk was
subjected to the same treatment: and the precipitation of gelatinous silica was
even more marked in this case than in the first.

There can be no reasonable doubt after these results that at a boiling-
temperature silica in some of its modifications is capable of replacing CO 2 to
a slight extent in concentrated solutions of alkaline carbonates and thus
passing into solution as an alkaline silicate. But we can scarcely infer such
action from these data in the case of the Soudan agates mentioned ; and we
are thrown back upon the action of decomposing organic matter (perhaps
when the drainage of the locality was different) as the most probable origin of
the solvent which corroded these agates.*

* In revising this work in its present form it may be as well to refer those
gentlemen, who take upon themselves to deny (in the name of "English
Chemists ") the existence of these acids, to the recent researches of C. G-.
Eggertz. (Biedermanris Central-Blatt far Agri. Ckem., vol. xviii, part 2.)
Such tricks of debate are worthy of the 'professional expert.'

106

Appendix

Supplementary and Explanatory Notes.

NOTE A.
Reduction and Dissociation in Volcanic Action.

Davy's well known suggestion that crateral explosions might result from
the escape of large bodies of free hydrogen, as a result of the reduction of
water (or steam) by the action of the metals of the alkalies and the alkaline
earths, is scarcely admissible, as it stands ; since at our highest furnace -
temperatures the oxides of these metals are not found to be dissociated, and
in the case of volcanic phenomena we have to add another factor, in the
retardation of dissociation due to the pressure that must exist in the volcanic
canal. Yet the idea which underlies Davy's suggestion contains more in it
than appears at first sight. The facts (1) that all the more basic lavas
contain a considerable quantity of the metals iron and manganese (not to
mention any others) ; (2) that their existence either as basic constituents of
volcanic minerals and as peroxides in the lavas as we know them, and which
we have no opportunity of examining in the volcanic canal, or in any stage
before their propulsion into the air and consequent further oxidation by
atmospheric oxygen, is no proof of their non-existence as lower (basic) oxides
or even as free metals (and therefore as reducing agents) within the canal ;
these facts taken together seem to point to the idea of Davy as worth more
consideration than it has generally received from geologists.

I have searched in vain for any recognition of the idea of Davy as here
elaborated both in Geikie's and Green's text-books, in neither of which does
it appear to be even touched upon ; and in a recent paper before the Royal
Society, Prestwich refers to Davy's suggestion only to dismiss it with a
passing remark or two. The late Prof. Phillips in his interesting and
suggestive work ( Vesuvius) in criticising Dr. Daubeny's views (p. 320) comes
so near the idea, that there can be little doubt that a little more knowledge
of thermal chemistry would have led him to see a reasonable cause for the
occurrence of great masses of " unmistakable free hydrogen in a blaze above
the mountain." The occurrence of such free hydrogen, would of necessity
lead to explosive combustions as it mixed with free atmospheric oxygen ; and
herein the brilliant and versatile mind of Phillips would probably have seen
a more efficient explanation of the production of great clouds of steam, like
piled-up bales of wool, to which he so frequently adverts in the work named,
than in the hypothesis of the ' sphseroidal state ' referred to in the text.
The fact of flame actually occurring in some cases, though not in all, is too
well attested (see Phillips, loc. cit. pp. 153,4) for us to be satisfied with a
mere dismissal of the subject as an 'illusion' Huxley, Physiography, pp. 190-
91. The same writer (loc. cit.) seems to confuse the products of combustion
with the combustible gases, which we have no means of directly examining
but spectroscopically. With this method we ought to be able to settle the
existence or not in definite instances, of glowing hydrogen at the volcano's
mouth ; and until its existence has been thus proved, it is perhaps not very
profitable to speculate as to its origin.*

There is a further point raised. We might ask the question whether mere
heat might not dissociate the steam contained in the glowing lava, as soon
as it was released from the pressure of the canal. Now we know (as I have
pointed out elsewhere, Chemical News, vol. liv. (1886, No. 1402) that the

* From a conversation with Dr. Johnston-Lavis during last year I gathered
that his observations of the eruption-phenomena of Vesuvius during recent
years do not lead him to assign any important part to the combustion of
hydrogen in the explosive phenomena observed.

APPENDIX li. 107

temperature of initial dissociation of steam lies somewhere between the
melting-points of silver and platinum ; but for the complete dissociation of
steam we require a temperature far above the melting-point of platinum,
and above the temperature even of the oxyhydrogen flame. How far such
conditions are realisable in a glowing lava issuing from a volcanic canal can
only be determined by an extensive series of observations ; but that an
approximation to such conditions is not improbable, may be seen from the
fact (Geikie, Text-book, p. 227) that, after a lava-stream has so far cooled
as to allow the observer to approach it, its temperature is found in some cases
to be, even near the surface of the mass, above the melting-point of silver.
If dissociation occurred in this way, we should it is hardly necessary to
point out refer these paroxysmal explosions to the presence of oxyhydrogen
gas, which, as I have often shown in a lecture-experiment, takes place in a
glass-tube at a feeble red heat of the glass.

Further systematic observation of volcanic eruptive phenomena may show
that there is some truth in both the explanations here suggested. To settle
which of these two hypotheses furnishes the more efficient explanation of
these crateral explosions it will be necessary to connect them with the
general nature of the ejectamenta. Should it turn out that the phenomenon
is associated only or principally with the basic ejectamenta, we shall have to
give the palm to the former the reduction-hypothesis ; on the other ha.nd,
should it be found to be associated equally with basic and acid ejectamenta,
the latter the dissociation-hypothesis would have to be recognised as
pointing to the more efficient cause.

In this discursus we are concerned only with the explosions and consequent
piling-up of a pillar of dense clouds ; the tension of confined steam, as the
main factor in bringing about the explosive phenomena, to which the clastic
products of vulcanicity are due, is another question altogether. A comparison
of my own observations on the abundance of tuff interbedded with such acid
lava-flows as those of the Dyas-period in the Rittner and Grodiier regions of
the Alps and the mixed nature of the highly basic ejecta of the Vesuvian
and Phlegrsean regions seems rather to suggest that this is common to both
kinds.

With regard to the possible occurrence of iron in the elementary state a
reference may be made to the observations of Nordenskjold on the west coast
of Greenland, in 1870. If iron occurred in anything like such quantities in a
molten lava in the canal of a volcano, it is clear that we should have an agent
present capable of reducing steam to free hydrogen on a grand scale; and
laboratory experiment teaches us that the oxide of iron formed in the
reduction of steam at red heat is Fe 3 64, which as magnetite is a common
constituent of basic igneous rocks.

It is difficult to conceive what the late Prof. Phillips could have meant by
the "cooling of water-bubbles below the spheroidal state to some red heat."
Could he have been thinking all the time of the "critical state" of water?
(See Vesuvius, p. 265).

NOTE B.

Vitality and crystal-building. When Prof. Judd's brilliant Address
appeared, in 1887, I took an opportunity of pointing out to him the visionary
nature of his speculation. I said "when the chemist shall have succeeded
in producing synthetically one particle of living protoplasm, we shall be able
to make a real departure in the direction indicated in the Address." The
importance of it may at once be seen by the emphasis laid upon it later in
the year both by Prof. Sir H. E. Roscoe, in his Presidential Address to the
British Association at Manchester, and by Dr. Schunck in his Address to
Section B, at the same meeting.

Again, the growth of minerals by accretion was fitly contrasted by Dr.
Woodward (in his Address to Section C), with the growth of living organ-
isms by assimilation and intussusception.

108 KOCK-METAMORPHISH.

'Or, once again, the trite phrase, "ohne Phosphor kein Gedanke," no
doubt expresses a truth, but not the whole truth. That phosphorus is an
essential constituent of the hard grey matter of the brain-centres is one fact ;
and that mental strain is shown by analysis of the excretory products to
involve a more than average expenditure of this, is another fact ; yet these
facts tell us absolutely nothing as to the state (high or low) of combination in
which the element itself exists in the living active organ of thought. All the
phenomena seem adverse to the hypothesis here criticised ; as those phases
of activity which we commonly call " vital " seem to be connected with
matter in a transitional state (waste and replenishment of the organ being
the necessary condition of vitality), in, so to say, a low degree of molecular
structure giving the freer play to atomic forces.

NOTE 0.
The Hypothesis of a metallic kernel.

Osmium the heaviest of all known bodies, (sp. gr. 22'48) and Ruthenium
with a sp. gr. of 12*26 (between those of lead and mercury) are even more
difficult to fuse than platinum, osmium having never yet been fused even in
the oxyhydrogen flame, Iridium again has a higher Sp. gr. than platinum.
Both osmium and iridium occur in platinum ores (in the un dissolved
residues of which they were first discovered) as alloys with that metal.
From considerations such as those advanced in this work, and from a
comparison of the sp. gravity of the globe with the specific gravities of
siliceous rocks, it would appear likely that these exceedingly dense and
refractory metals and alloys were the earliest of all known forms of matter,
to condense and form the solid core or nucleus of the globe ; and it may be
that some of our so-called " rare " metals are only rare at or near the earth's
surface. It is even possible that other and even denser forms of matter
entirely unknown to us may exist. I have elsewhere discussed the two
factors, conduction of heat and pressure, in relation to the possible existence of
materials in a state of fluidity at intermediate depths. See British Associa-
tion Report, Birmingham Meeting (1886), pp. 657,8.

NOTE D.
On wet and dry reactions.

The old adage " corpora non agunt nisi soluta " is perfectly well known to
the modern chemist to be a formula for a generalization based on insufficient
data, which is only a partial expression of the truth that molecular eontact is
a necessary antecedent of chemical action. This is the simple explanation of
such bodies as, for example, dry sulphur and copper (quoted by Mr. Harker in
his essay at p. 847) being made to combine under great pressure by
M. Spring.

The truth, in its more general range, may be illustrated by a very simple
experiment. Mercuric chloride (HgCla) reacts upon potassic iodide (KI), as is
well known. If 4 gins, of the former and 5 gms. of the latter be dried
separately at 100C in the form of coarse powder, and roughly mixed by
shaking round in a dry flask, a certain pinkish hue, due to the formation of
mercuric iodide [HgCl 2 + 2KI = HgI 2 + 2KCl] is at once perceptible in the
mixture. This deepens to a bright pink red when further molar division of
the mixture, effected by rubbing it in a mortar, increases the number of
points of contact of the solid particles (so marked is the colour that it may be
exhibited to a large class in a lecture). But even this is feeble as compared
with the instantaneous production of the intense lovely hue of HgI-2 obtained
by mixing together aqueous solutions of the two salts previously prepared
separately in the ratio indicated above, which is an approximation to that of
their 'equivalent proportions.'

APPENDIX ii. 109

NOTE E.
Sedgioick't Hypothesis as to ' Waves of Heat. '

As an instance of the unscientific play of the imagination we may note the
imaginary basis of a theory proposed by even the great Sedgwick, when he
conceived that " both cleavage and foliation are due to the parallel trans-
mission of planes or waves of heat, awakening the molecular forces and
determining their direction " a view endorsed (as it appears) by Prof. H. D.
Rogers. The unreality of the basis on which it rests is seen at once by
bringing it to the test of a few simple physical principles, based on un-
questionable experimental data.

We know of three ways in which heat is transmitted :

1. Radiation,

2. Convection (in fluids),

3. Conduction.

The first is clearly out of the question ; the second is equally inapplicable to
a solid mass ; the third is nothing more than a tendency towards equilibrium
in the heat-energy contained in a system, by virtue of which when masses of
matter are in contact at different temperatures, there is a constant transfer
of heat from the hotter to the colder masses until equalization of temperature
throughout the system is attained. But the propagation of a series of ' waves
of heat,' would require also the converse of this ; which is about as possible
as that water should flow up-hill under the mere influence of gravitation.
The apparent analogy of the propagation of sound-waves in such an elastic
medium as air (coupled with the obsolete caloric-hypothesis) where the
alternate condensation and expansion of a material body form the counterparts
of each other in every undulation, probably misled Sedgwick.

NOTE F.

The terms 'vitreous' and 'amorphous.'

It will be seen that in this work the term ' amorphous ' is used in a more
limited sense than is common among mineralogists, with whom it is
customary to include those forms of matter which are here distinguished as
' glassy ' or ' vitreous.' (See Rammelsberg, Handbach der Mineral chemie,
pp. 38 40). That authority defines (p. 38, loc. cit.) amorphism as follows:
' Die Masse eines festen Korpers wird amorph genannt, im Gegensatz zu
krystallisirt, wenn die Kennzeichen des krystallisirten Zustandes ihr fehlen."
The facts considered in the thesis in connexion with devitrification have
rendered it necessary to adopt the nomenclature of chemistry rather than of
mineralogy ; since its aim is rather to get at the principles the natural
operations which determine those phenomena, the description of which is the
proper function of the pure mineralogist At the same time the difficulty of
drawing a sharp line of distinction between non-crystalline bodies is not
unperceived, since these graduate from the condition of a true glass through
the various phases expressed by the terms 'opalescent,' 'gelatinous,' 'flocculent,'
to that which is implied by the term 'amorphous,' as it is used in chemistry.
The following examples of variations in spec, gravity with allotropic forms
given by Rammelsberg (loc. cit p. 39) may be compared with the cases cited in
this work :

Amorphous Crystalline

Sulphur 1-92

Selenium ... 4'28

Phosphorus 2'18

Silica 2-20

Antimony sulphide ... ... 4 '28

1-96 2-07

475

1-82

2-3 and 2'6

4-60

2-56

Orthoclase 2'34

On this table it is to be noted :

(1 ) The lighter form of ' crystalline ' sulphur given as having a specific
gravity of 1'96 is the sulphur-glass of the prismatic crystallites, which have

110 BOCK-METAMOKPHISM.

generally (I believe always) been hitherto described by chemists as ' crystals ; '
upon the strength of which sulphur has been spoken of (as it would
appear erroneously) as a 'dimorphous body,'

(2) The same confusion appears in the case of phosphorus, and introduces
what appears at first sight an anomaly, which, were it anything more than
an apparent exception, would vitiate the generalization enunciated in this
work ; but on the other hand causes no difficulty, when it is seen that it is
in reality the vitreous form of phosphorus which has the density of T82.

(3) The 'amorphous ' orthoclase is really the glass.

The facts are quite sufficient to justify the distinction which has been
drawn in the text of this work between vitreous and amorphous bodies in
the chemical sense.

From recent observations I have been led to recognise vitreosity as a
phenomenon occasionally exhibited by water under conditions favourable
to rapid loss of heat at OC. (See Nature, vol. xxxvii, p. 104).

The facts cited certainly lend support to the view advocated in this work
as to latent heat of vitrification ; and the fact of the greater brittleness in
frosty weather of ordinary window-glass which is a matter of common
observation may perhaps be explained on the same principle. Further
investigation may possibly also lead to our recognition of vitrification as the
real explanation of the silvery blue tint which characterizes 'glacier-ice.'

NOTE G.
On Fritting.

Since the suggestion as to the possible formation of Wollastonite by
direct replacement of C0 2 by SiO 2 in the dry way was written, I have had
the privilege of seeing in Dr. Percy's collection a fine specimen of Wollastonite,
made some years ago in his laboratory, by heating finely powdered calcite
and fine clean quartz sand together in equivalent proportions. On turning
to p. 46 of the new edition of the volume on Fuel, &c., by that distinguished
metallurgist, we find the rule laid down, that ' Silica combines readily with
a metallic oxide, when an intimate mixture of the two is heated to the right
degree, provided the oxide be not reducible, per se, at or below the temperature
required for combination.' And he points out that fusion is not necessarily
required for combination (e.g. in the case of silica and lime), and in metal-
lurgical operations in the process known as 'fritting,' is to be avoided
with an easily fusible oxide of a heavy metal (e.g. protoxide of lead).

The occurrence of Wollastonite as a product of contact-metamorphism has
been recorded by Prof. Heddle in the marbles of the north-western portion of
the Scottish Highlands. (See Q.J.G.S., vol. xliv, p. 411).

The fact that ' slags ' are composed of silicate of lime and other silicates
formed in the dry heat of the furnace, is most important in its petrological
bearing ; and must always be borne in mind as a caution on instituting any
comparison between them and the silicates found in the crystalline rocks,
the schists on the one hand, and the eruptive rocks on the other. This
consideration too tends to confirm the theory advocated in this work, that
the original minerals of the schists have crystallized in the presence of
superheated water, though not ' precipitated ' in aqueous basins.

NOTE H.

Orograpkic Structure of the Alps.

The importance of looking at the structure of a comparatively young
mountain-system is seen from the fact that most of the mountains of these
islands are but the worn-down stumps of ancient mountain-systems. Non-
recognition of this fact has it appears, led to some erroneous theorizing
among the untra veiled members of the British fraternity of geologists : they
have mistaken the abnormal for the normal relation of things, and have
been in consequence a little too ready to arrive at advanced theories as to

APPENDIX ii. Ill

' regional metamorphism.' The recent publication of Prof. Heim's great
work Mechanismus der Hocligebirgsbildung has opened their eyes to some
leading principles which have been for some years more or less patent to the
leading geologists on the continent (Switzerland, Austria, Germany). I
shall sketch here briefly a few of my own observations made during several
traverses of the main or central chain of the Alps in past years (chiefly from
1877 to 1883) ; premising that the general orography of the Alpine system
a central chain of crystalline rocks with flanking chains of sedimentary
rocks, (mainly of Secondary and Tertiary ages) is known to everyone who
takes an interest in this subject.

My first traverse was made from Fliielen by Altdorf, Amsteg, and the
Kreuzli Pass to Sedrun and Dissentis in the Vorderrheinthal. This pass is
somewhat rough and unfrequented, but has the advantage (to the geological
observer) of being free from glaciers. One could not fail to be struck, as one
penetrated the mountains from the north, with the distinct succession of
slates, schists and gneiss into the alpine granite which seemed to form the
backbone of the range ; the same succession (in part) being seen in reverse
order as one descended to the V. Rheinthal, and in a walk down the Reussthal
another time from Andermatt to Amsteg. The valley of the Vorder Rhein is
apparently formed by a gigantic flexure in the main chain, which has left the
oldest rocks covered up by a series of younger schists of undetermined age,
including the Bundner* and Casanna Schiefer (the "schistes gris" and
"schistes vertes" of the Swiss geologists), and some later formations,
including the Verrucano. Leaving the main valley at Ilanz, and striking up
the side valley to the south, one passes through these younger phyllites and
schists, all the way up the valley to Vals-am-Platz. Near Furth I found
talc-schist, and higher up the valley the green schists (schisteverte)
and marble (calcaire indetermine') interbedded with the grey schists
(schists gris). I have twice traversed this valley, and have thus repeated
the observations. The second time one had rather exceptional advantages
from the fact that a new road was in process of construction, and the
blasting-works had laid bare large exposures of fresh rock-surface. The
foliated structure of these rocks at a very high angle has facilitated the
erosion by the mountain stream of most magnificent gorges, quite compar-
able with that of the Via Mala for depth and narrowness. On my first
traverse the Pass of the Valserberg was covered with snow ; but on the second
occasion it was free from it ; and I observed what appeared to be the green
schists forced in contorted masses into the older schists. f On referring
to Studer and Escher's map I find the green schists and marble shown as
cutting through this range into the Hinterrheinthal, between the older
mica-schists and the grey-schists. These last are met with down the Valley
as far as Spliigen. In the road cuttings between that place and Andeer
the mica-schists and gneiss are well exposed, and have a decidedly older look
than the schists of the Valserrhein. Two different routes were taken on the
two several traverses from the last-named place to Bivio-Stalla, at the foot of
the Julier Pass.

On the first occasion the wild steep rough valley of the Averser Rhein was
followed up to Cresta (the loftiest Swiss village according to Badeker).
The lower part of this valley has a wildness and ruggedness almost peculiar to
it in my experience ; and on referring to the map, this seems to be accounted
for by the fact that it is cut through the oldest schists and gneiss which

* Local geographical name (Von Hauer) from the territory of the mediaeval
'Bund.'

t Field observations on these green schists of this region of the Alps
certainly suggested even at that time (1878) an igneous and intrusive origin
for them. (cf. Kalkowsky, op. cit., p. 217). They are probably the
Valrheinit ' of Rolle,

112 KOCK-METAMOEPHISM.

are much more indurated than those of the younger series of schists
developed higher up the valley. Mining for metallic ores is carried on in
the older series. Crossing over the Stallaberg, dykes of green serpentine
with a pseudo-fibrous cleavage are seen cutting through the mountain and
well exposed by the side of the mountain-path which crosses them.

On the second occasion the route was from Andeer by the Via Mala to
Thusis ; thence by the Schyn Pass, Tiefenkasten, and the Oberhalbsteinthal
to Bivio. The deep-cut steep gorges of the Schyn reminded one of those of
the Valser Rhein, and this character the two valleys seem to possess in
common from the fact that they are both cut into the younger schists, as in
fact is the gorge of the Via Mala, where the rocks are of a more calcareous
character ; an instance of these grey schists becoming in places massively
kalkhaltig, (Giimbel). Green and red serpentine is so abundant in the
Oberhalbsteinthal as to be used commonly for road material ; and about
Miihlen this mineral appears to permeate the 'green schists.' I obtained
opposite the hotel at this place a specimen of a highly-contorted portion of
these schists, with marked foliation. Ascending the Julier we have (1)
Casanna Schist, (2) Gneiss, (3) Granite (intrusive) ; and portions of the
schists are found above the Pass ; while lower down, on the south side, a
quartzite (probably grauwacke) is quarried by the road-side. These appear
from their position to be portions of once-larger masses infolded in the
granite and gneiss in the process of mountain-building. I have studied the
gneiss and schist of the Upper Engadine from the Maloja Pass to Samaden,
and have been impressed with the strong evidence in them of some sort of
bedding. The serpentine too of the Oberhalbstein was observed cutting
through into the valley above Silvaplana. The granite of the Roseggthal,
the gneiss about Samaden and St. Moritz, the granite and gneiss of the
Bernina Strasse and the schists of the Alp Griin have all been studied ' in the
field.'

On one occasion I returned from the Engadine by the Suvrettathal and
the Val Bevers, then over the Weissenstein granite massif, dropping down
to the Albula Strasse near the top of the pass. Walking down to Bergiin,
good road-sections were seen in the Liassic slates, the cleavage of which was
so pronounced, that the eye could follow its strike through the mountains to
the north west. On petrological grounds alone, and quite independently of
their organic remains, I should say there was no difficulty in distinguishing
these slates from the phyllites of the older (archsean) series.

From Bergiin Davos-am-Platz was reached by the Sertig Pass and
Frauenkirch, the older schists being seen in the lower part of the Sertigthal.
From Davos Landquart was reached by the road through the Priittigau, the
valley traversing the younger or grey schists and phyllites. On another
occasion the same series of schists and phyllites was studied in a journey
from Tiefenkasten by Lenz and Churwalden to Chur.

Impressions strongly forced in upon one's mind in these travels and
observations were such as the following :

(1) The fact of a general order of succession prevailing in the oldest and
most highly ' metamorphosed ' strata of the Alps ;

(2) The existence of more than one series pf schists, those of some
districts having altogether a younger and less indurated character than
those of other districts where they are directly related to the central or funda-
mental gneiss ;

(3) Frequent strong signs of some sort of bedding, even down to the gneiss ;
marked examples of which were observed in the Reussthal and on the Maloja ;

(4) Signs in some cases of subsequently-induced subordinate structural
characters, such as cleavage and the deposition of veins of secondary quartz.

Of the rapidity with which the younger phyllite series undergo degrada-
tion by atmospheric agents in weathering, as compared with the older schists,
I have noted several illustrations, in the work done by great downpours of
rain in the mountains. For example, a storm during the previous night

APPENDIX ii. 113

had swollen the waters of the Nolla which rolled like a sea of black mud into
the Rhine at Thusis ; and about half-an-hour from that place the road to the
Schyn Pass was found blocked by a vast mass of debris from the same black
slaty rocks of the phyllite series.

Again in wandering in the Bavarian Alps, in the Vorarlberg, and in the
Salz Kammergut on the north side of the Alps ; in contrasting the calcareous
series of the north side of the Ennsthal with the micaschists on the south
side ; in contrasting the Triassic strata of the Dolomite Alps to the south of
the Pusterthal with the phyllites the mica schists and the gneiss of the
central chain on the north side of that valley ; and lastly in following the
latter up the valley from Lienz to Heiligenblut and on to the Gross
Glockner, one could hardly help reflecting upon the slight petrological
change wrought in the later and undoubted sedimentary series with the
enormous degree of ' metamorphism ' which the rocks of the central chain had
undergone, on the assumption that they owe their present character
altogether to ' metamorphism.' This is especially striking when one compares,
in hand -specimens or in field-sections, the Triassic limestones on either
the north or south side with the massive calcareous schists (Kalk-glimmer-
schiefer) which are seen interbedded with the chloritic schists and above
the mica-schists of the Glockner, * (exposed in good fresh sections below
the Glockner House.) The slight splintery kind of incipient cleavage, which
has given to the ' Dolomites ' on the south side and to ranges like the
Donner Kogeln on the nor thside above Gosau their weird and grotesque
appearance (as the result of the peculiar mode of weathering caused thereby
in these rocks), and is well seen in hand-specimens of some of the Hallstatt
limestones (which have undergone a certain amount of metatropic change
in places into marble), is altogether a different thing from the marked
foliation observable in the Kalkglimmerschiefer of the Glockner district.

Reflecting on such facts, one found oneself gradually driven of necessity
to conclude that the mere pressure, which has wrought so small a petrological
change in the sedimentary strata of the flanking ranges of the Alps,
cannot be accepted as the cause of the petrological characters peculiar to the
gneiss and schists of the central chain. Some of the extreme results of such
pressure are seen in the true cleavage of the Glarus slates (Dachschiefer)
where the Eocene strata have been subjected to excessive pressure between
two great overfolds of the older strata (Doppelfalte,) t and of the slates
of some of the secondary formations of the Alps (as in the case cited in this
note and other well-known instances) ; and I have a specimen of rock from
the Schafberg above Pontresina (probably originally a quartrite) in which a
rough cleavage-foliation cuts right across the bedding. J No one would contend

* Position assigned to them by Giimbel. (Anleituny zu wiss. Beobach. auf
Alpen-reisen, fig 52), cf. Bonney, (Q..T.G.S. vol. xlv, pp. 86-90).

tSee Heim, Mech. der Gebirgsbildung, (Atlas, prof, v, vi, vii, viii.) In the
text of that work (Bd. i, pp. 143-146) Heim has given in full the palaeon-
tological and lithological evidence of the Eocene age of these Glarus slates.

JThe "quartzite in which a rough cleavage-foliation" has been developed
appears to be a schist, composed of alternate layers of a brownish
yellow mica and quartz, the mica occurring in very thin layers, little more
than films, under the microscope. The quartz has the 'mosaic-like' texture,
and some of it appears to have taken the form of tridymite, as if the rock
had been partially fused. On the other hand many of the quartz grains
strongly suggest a clastic origin for the original rock. These facts and the
proximity of the rock to the granite seem to suggest that the character of
the rock is the result of contact-metamorphism. It is also intimately
associated in the same quarry with a schistose chloritic rock, which
as seen under the micr. must I think be certainly considered a product of
contact-metamorphism. There is an accessory mineral scattered pretty
I

114 BOCK-METAMOBPHISM.

I think, that the lower degrees of metamorphism which these rocks exhibit
can bear any comparison with the characters of the schists and gneiss of the
oldest Alpine series. To the observations in the Alps which have been
sketched in this note, may be added others, followed up however in less detail,
in the Erzgebirge, in the Dresden and Freiberg country, in Thiiringen, and
elsewhere. Referring these to the horizontal ground-plan of those parts of
Europe as indicated in the maps of Von Dechen, Studer and Escher, and Von
Hauer, one's mind has been educated to appreciate the simplicity of the idea
which runs through the tabulation of the rocks by those eminent geologists.
In this way one has been led to view the crystalline archaean rocks, as
nothing more nor less than portions of an original earlier crust of the globe,
which have worked their way up through the later crust, under the influence
of those great lateral strains, to which the crust as a whole has been subjected
in the contraction of the terrestrial mass from loss of heat ; while there is no
ground for supposing that in such movements these so-called highly meta-
morphosed rocks have suffered a higher degree of change than those minor
phases (metataxic chiefly) which can be seen in the undoubted sedimentary
strata that have partaken in the same movements.* It follows further from
this that the crystalline archozan rocks (gneiss, and schists, and phyllites) owe
their mineral character (their essential morphology) to causes independent of
the subsequent movements in which they have taken part ; that is to say, to the
conditions under which they were originally deposited.

NOTE I. (cf. p. 10).
On Serpentinization, &c.

The following note (based on data furnished by Rammelsberg's ' Mineral -
chemie') may be useful as illustrating the chemical changes involved in
serpentinization and some allied processes :

OLIVINE FeO + SiO 2 + H 2 O = SERPENTINE.

[MgO, FeO.] [MgO (FeO) + H 2 O.]

An isomorphous mixture of ^-silicates of MgO and FeO, by losing a good
portion of its ferrous oxyde and taking up silica and water, becomes essentially
a hydrous silicate of MgO.
AUGITE - CaO + H 2 O = SERPENTINE.

[CaO, MgO, (FeO, A1 2 O 3 )]

[or~

HORNBLENDE - CaO ( - FeO) + H 2 = SERPENTINE.
[MgO, CaO, FeO (A1 2 3 )]

In the second and third cases some A1 2 O 3 would probably remain as an
impurity mechanically mixed with the mineral. Kalkowsky (' Lithologie,' p.
208) remarks on the serpentinization of amphibolites which are poor in A1 2 O3,
the hornblende passing into a clear green fibrous magnesia-hydrosilicate.

generally in well-individualized particles through the specimen ; and these are
generally elongated in the direction of the foliation-planes. In places it lines
the walls of, and in one place fills up, a branching fissure or former crack in
the slide which runs obliquely across the foliation, and is filled for the most
part with secondary Si0 2 . The mineral is quite opaque and is probably
magnetite. Glassy quartz is visible macroscopically in thin folia. Conversion
of quartz into tridymite requires the temperature of a porcelain-furnace, to
melt it into opal that of the flame of the oxyhydrogen blow-pipe (Rammels-
berg, Mineralchemie, p. 162.)

The statement of Kalkowsky which has been already quoted (Lithologie, p.
270), seems to apply to this case ; so that it would furnish an example of what
that writer terms 'Quartzitschiefer.' It is rather a nice question perhaps
whether the case is one of anything more than 'cleavage-foliation.'

* Mr. Marr has arrived at a similar conclusion in the case of the so-called
argillaceous schists at Ilfracombe, where he recognizes 'sedimentary rocks
possessing all the mechanical peculiarities of normal schists, without any
great amount of chemical change.' (Nature, voL xxxvi, p. 591).

APPENDIX ii. 115

GARNET (reap. Thongranate) - CaO -f MgO + H 2 O = SERPENTINE.

[ Al a O 3 + FeO or CaO] [ + A1 2 O 3 as an impurity.]

Further

Serpentine - FeO = Talk.
Serpentine - MgO + O = Haematite.

On the other hand, with a different rock-environment furnishing a different
set of minerals in solution, the chemical changes set up would follow a rather
different direction, thus :

Olivine - FeO + H 2 = Chlorite.
Oli vine -MgO f- H 2 O = Chlorophaeite.
Augite (in some cases) - CaO + H 2 Chlorite.
Chlorite (with slight chemical changes) becomes Serpentine.
With the common conversion of olivine (one product of dry fusion) into
serpentine by hydrochemical action it is interesting to compare the relation
which Pectolite bears to another product of dry fusion, the normal silicate of
lime, Wollastonite (Ca SiO 3 ).

On the other hand, from a series of anhydro-chemical reactions of a mineral
magma upon the normal silicate of magnesia, Enstatite (MgSiOs), such
minerals as Bronzite, Hypersthenite, Diopside, may have originated (see
Rammelsberg, op. cit., pp,* 379-389.)

Roth (Allgem. u. Chem. Geol., Bd. i, p. 67 ) enumerates the non-aluminous
silicates Olivine, Enstatite, some Augites and Hornblendes, Diallage, Chon-
drodite, as the more important minerals convertible into serpentine (and its
allied minerals), which often replaces them in pseudomorphs. (cf. Note N.)

NOTE K.

Pfaff (Allgemeine Geologic als exacte Wissenschaft) has to some extent
anticipated me with reference to the conditions of the earth's surface, under
which the earliest ' rocks ' must have been formed ; conditions which I have
in this work deduced independently from known chemical and physical facts
and principles. See also British Association Report, Birmingham Meeting
(1886), pp. 658-60. Pfaff's doctrine must be stated in his own words, p. 143

"In den friiheren Zeiten war stets eine Granzlinie der Tiefe vorhanden,
tiber welche hinaus kein flussiges Wasser gelangen konnte. Diese Granzlinie
riickte mit der fortschreitenden Erkaltung immer weiter nach abwarts, und
von einem gewissen Zeitpunkte an trat dann das Verhaltniss ein, welches wir
jetzt finden, dass nehmlich flussiges Wasser bis zu dem heissfliissigen Erdkerne
selbst dringen kann."

NOTE L. (cf. pp. 21, 92).
The genesis of the Diamond and of Graphite.

In the year 1880 Mr. Hannay announced that he had succeeded in making
real diamonds by the reducing-action of sodium upon hydrocarbons at high
temperature and pressure.* Assuming that the specimens submitted by him
to Prof. Maskelyne were made in this way, much new light would seem to be
thrown upon the obscure process whereby the diamond is formed in nature.
The presence of heat and pressure in the depths of the earth's crust sufficient
for the purpose will be readily admitted ; the real difficulty which remains is
to find a sufficiently energetic electropositive metal to do the work of reduc-
tion, by taking up the hydrogen of hydrocarbons. There is some considerable
difficulty on general grounds in postulating the presence of the alkalimetals,
but less difficulty in considering that the metals of the alkaline-earths might
be present. Looking however at the position of magnesium (in the electro-
positive order of the metals), it seems probable that this widely-distributed
metallic base of silicates might be regarded as the most likely metal to have

* See Nature, vol. xxii, p. 255.

116 ROCK-METAMOBPHISM.

effected the reduction ; and this hypothesis is strengthened by some of the
known chemical properties of magnesium. In this way I had, on purely
theoretical grounds, anticipated the conclusion at which Prof. H. Carvill
Lewis arrived. (See Brit. Association Report, 1887.)

Messrs. Roscoe and Schorlemmer (Treatise on Chemistry, vol. i. p. 582)
mention the occurrence of dark spots in diamonds, which Brewster considered
to be cavities, but which Sorby has shown to consist of " small crystals of
much lower refractive power than the diamond itself. " May not these be
' belonites,' and as such traces of an incomplete metatropic change of the
carbon ? An affirmative answer to this question is certainly suggested by the
further fact noted by the same writers, that " Goeppert has noticed in certain
diamonds the occurrence of a cell-like structure resembling that obtained
when a jelly undergoes solidification." To the statement of Roscoe in his
Elementary Chemistry (edition 1887, p. 75), that the diamond "cannot have
been produced at high temperature, because, when heated strongly in a
medium incapable of acting chemically upon it, the diamond swells up and is
converted into a black mass resembling coke," we must demur, as the effect of
the simultaneous action of great pressure is ignored.

With this we may connect the occurrence of Graphite in the earliest rocks.

Some stress has been laid upon this fact by different writers, as indicating
carbonization of organic matter ; and then the further inference is drawn
that the rocks in which it is found must have once been sites of vegetable
growth, or derived from other rocks which were. If this were a necessary
inference it would certainly lend strong support to the extreme theories of
metamorphism which have been maintained in some quarters. We must
see if experimental chemistry can throw any light on this subject. (1) We
know perfectly well that 062 can be reduced to elementary carbon in the
purest form by hot alkali-metals, and even by heated metallic magnesium ;
but this introduces the further difficulty of explaining how such metals
could have existed in any quantity in the free state, since the alkali-metals
must have been among the earliest elements to undergo oxidation ; and the
avidity with which they attack water to form hydroxides makes it still more
difficult to understand how they could have existed in the free state at that
stage of the earth's evolution at which we consider these rocks, in which
graphite occurs, to have been formed. But (2) we know also that at high
temperatures elementary carbon can combine directly with hydrogen, a fact
which we frequently call into play in the laboratory in the synthesis of
acetylene gas (CaH 2 ) at the temperature of the electric spark-stream. Here
then we have a clue to the mystery. Anyone who is familiar with the
chemistry of the hydro-carbons will, I think, be prepared to admit that such
bodies might have been formed in some quantity under the physical conditions
existing at the time when the archsean rocks were formed.* Given the
presence of such bodies in the nondifferentiated mass from which the materials
of these rocks were deposited, how could they be reduced to elementary
carbon by the abstraction of their hydrogen ? In two ways : (a ) by direct
dissociation of the higher hydro-carbons ; marsh-gas and olefiant gas (e.g. )
undergoing this change before our very eyes in the eudiometer under the
influence of the high temperature of the spark -stream, and depositing carbon
on the terminals with such rapidity as to repeatedly bridge over the interspace
and close the circuit ; (b ) by actual reduction under great pressure and high
temperature, by the chemical action of strongly positive bases. Perhaps the
nearest approach to the physical conditions under which we conceive graphite
to have been formed in the fundamental rocks is found in the heated interior
of the ordinary fire-clay retorts in use in gas-works. " A very pure and dense

* Do we know absolutely anything as to the origin of the vast stores of
hydro-carbons in the oil-fields of America, or of that of the greater stores of
the Baku region ? Are there any incontestable facts which go to establish an
organogenic, in contrariety to a cosmic, origin of those hydro-carbons ?

APPENDIX ii. 117

variety of carbon is found in the roof of old gas-retorts, where it has been
gradually deposited by the action of the high temperature upon the [hydro-
carbons of the] coal-gas which was passing out." * To the influence of high
temperature here mentioned I should consider " contact-action " of the heated
porous body of the wall of the retort as an important aid towards dissociation
and consequent deposition of the carbon.f All this is quite conceivable as.
taking place at an early stage of the evolution of the earth.

From these considerations it appears that there is no necessity for regarding
the presence of elementary carbon in the form of graphite in the archsean
gneisses and schists as indicating pre-existing organic matter.^

The interpretation of the presence of graphite in the archsean rocks as
indicating pre-existing vegetation was admitted even by Prof. Mobius (Der
Ban des Eozoon Canadense, ix) so late as the year 1878. His words are:
" Vielleicht riihrt der Graphit der Urgneissformation von Organismen her."
Kuntze in commenting adversely (Nature, August 28, 1879,) on Mobius'
conjecture as to the origin of graphite, denies its phytogenic origin in the
Laurentian rocks, though he does not take quite the line ofe argument which I
have in this Note. His objections are of a general nature ; and his strongest
point is the absence of water in the minerals of the archsean rocks, though
perhaps he overstrains this point a little. Of the existence of Kuntze's letter
(though I may have read it at the time of its appearance and forgotten it) I
was quite unaware while engaged in working out the view which I have
ventured to put forward as the probable explanation of the occurrence of
graphite : and I only came upon it in looking up afterwards such notices of
the Eozoon Canadense controversy as might have appeared since 1878. This
was in .April, 1888. So convincing to some minds has the presence of
graphite appeared as an indication of the pre-existence of vegetation on the
Earth, that in the Report of the Smithsonian Institution (1869) quoted by
Sterry Hunt (Chemical and Geological Essays, p. 302) the presence of graphite
even in aerolites is said to "tell us in unmistakeable language that these
bodies came from a region where vegetable life has performed a part not
unlike that which [it] still plays on our globe !"

Lockyer's experiments on meteorites (see Nature, vol. xxxiv, p. 280) are of
great theoretical interest in this connection. Experimenting on meteorites in
a vacuous space with a low -temperature spark-stream he obtained "the same
spectrum of hydro-carbons which Huggins, Donati, and others have made us
perfectly familiar with in the head of a comet." This seems to support the
hypothesis advanced in this Note as to the probable existence of hydro-carbons
in the archsean atmosphere of the Earth.

With a high-temperature spark-stream he obtained in a similar manner the
hydrogen spectrum, without the carbon spectrum. Is not this due to the
temperature of the spark-stream being in this case above that of the
dissociation of the hydro-carbon while below that required to give the
spectrum of incandescent vapour of carbon? If Mr. Lockyer will repeat this
experiment for some time and examine the apparatus afterwards for
amorphous carbon- dust, and also examine the spectrum of the spark as it
passes between two carbon-points in the synthetic formation of acetylene gas,
some interesting results may very likely be obtained tending to give a definite
answer to this question.

* Williamson : Chemistry for Students, 54.

t See Chemical News, vol. liv, No. 1402, where I have discussed this subject
at length.

Since this note was written, it has been announced in Nature, (Dec. 15th,
1887), that "carbon has been found between the laminse" of the great mass
of meteoric iron which fell near Cabin Creek, Johnson's County, Arkansas,
March 27th, 1886. Graphite "identical in properties with iron -graphite "
was also identified by Berthelot in the meteoric mass which fell at Cranbourne
near Melbourne in 1861,

118 BOCK-METAMORPHISM.

Etheridge (Pres. Address, 1881, p. 26) remarks: "The presence of graphite
in large deposits, occurring both in beds and veins in the Laurentian rocks,
clearly determines that its origin and deposition were contemporaneous with
the mass of the containing rock; the graphite, again, is associated with
calcite quartz, and orthoclase."

Sterry Hunt says (Ch. and Geol. Essays, p. 216 ): "Graphite, which itself
encloses apatite, is found included alike in quartz, orthoclase, and pyroxene,
and in calcite, in such a manner as to lead us to conclude that the
crystallization was contemporaneous with that of all these minerals." The
assumption of the necessarily phytogenic origin of graphite seems to have
found pretty general acceptance with recent writers of text-books (see Geikie,
p. 639).

In this Note I have done no more than drop a hint that the dissociation of
hydro-carbons (previously formed as they may have been by direct combina-
tion of hydrogen with glowing carbon-vapour) was greatly facilitated by the
contact-action of solid portions of a crust formed locally on the surface of the
magma, but still at. a high red-heat temperature, from a comparison of those
instances which I have discussed in the paper referred to in the note, and
which are within the range of our ordinary laboratory experience. I venture
to go further now, and to make the suggestion that this has been the main
agency concerned in the great deposits of archcean graphite. All that we should
seem to require would be the presence of slaggy solid portions of the Earth's
surface sufficiently cooled to be below the temperature of the evaporation of
carbon, which must be enormously high. Graphite thus becomes a rough
indication of the stage at which a hard but hot crust began to form locally on
the surface of the magma ; a beginning (that is to say) of the passage of the
Earth from the 2nd to the 3rd of Zollner's phases.

On May 12th, 1888, I subjected this theoretical view to the test of
experiment. A piece of ordinary combustion glass tubing about 8 inches long
was drawn out at one end nearly to a point, so that a continuous flame of gas
might burn at it, to answer the double purpose of guaranteeing that no
atmospheric air passed back into the tube at any stage of the experiment, and
giving at the same time a rough index of the rate at which a stream of gas
passed through the tube. The tube was filled with fresh-broken and calcined
lumps of pumice, and the wide end fitted with an ordinary wooden cork and
connexion-tube. This was then attached to an ordinary flexible tube for the
supply of common coal-gas. The coal-gas was passed through and ignited at
the other end. The tube itself was then raised to a dull red heat by a
Bunsen-flame about 6 inches wide ; and as I stood and watched it I had the
satisfaction of seeing the pumice-fragments gradually coated over with a black
deposit, possessing the dullish metallic lustre of graphite. It is impossible
that this could have been formed in any other way than by the dissociation,
through the contact-action of the heated porous fragments of pumice, of the
hydro-carbons of the coal-gas.

In order to further verify this, the tube was detached from the coal-gas
supply, and attached to a gas-holder full of oxygen, with the intervention of
drying-apparatus. The oxygen was passed for a few minutes (cold) through
the tube and then through lime water without any visible result; then a
narrow zone was heated with a flame of a spirit-lamp while the oxygen
continued to pass. The combustion of the carbon in the heated zone of the
tube was soon proved by the free deposit of carbonate of lime from the lime-
water, and in two or three minutes those pumice-fragments which were
exposed to the heat of the flame were perfectly cleaned to their native
whiteness. The theory of dissociation by contact-action thus receives visible
demonstration; and the application of the theory to the production of
graphite from hydro-carbons seems to be fully warranted.

I have since repeated the experiment on a larger scale, and have a tube full
of pumice-fragments coated over with carbon, more than 3 feet in length. In
this experiment I observed the deposition of elementary sulphur, from the

APPENDIX ii. 119

dissociation of the sulphur components of the coal-gas, along with the
deposition of the carbon.

Dissociation by contact-action at a pretty high temperature of portions of
the solid crust upon hydro-carbons may be regarded, I think, in the light of
such evidence, as a sufficient explanation of the occurrence of graphite in
rocks of undoubted archaean age; this therefore need no longer have a
phytogenic origin ascribed to it. As a simple corollary to this, it follows that
later, say in the stage represented by the Huronian of Prof. R,. D. Irving (op .
cit.) we should expect to find, under the conditions which he has deduced from
his masterly studies of that great group, that, the temperature being too low
to induce dissociation, the hydro-carbons would be deposited. Such 'hydro-
carbonaceous' material need not therefore be regarded (as he supposes) as
'organic matter' (p. 373) any more than the archsean graphite; and so his
argument on this point seems to break down. As for the iron-carbonates of
that group, they simply tell us that (on the assumption of their contempora-
neous origin) in the Huronian period of the Earth's dev elopement, the
temperature in that region was not above the dissociation-temperature of
carbonate of iron.

While this was passing through the press, Mr. T. Davies of the British
Museum of Natural History was good enough to direct my attention to some
very interesting specimens of graphite from Siberia, which are contained in
the National Collection, as exhibiting what had appeared to some to be traces
of 'organic structure.' From an inspection of them which through Mr.
Davies' obliging courtesy I have been able to make, I am convinced that there
is no trace whatever of organic structure in them; and that the phenomena
which they present can all be explained as the developement of a crude
prismatic structure caused by compound cleavage due to compression
combined with 'Answeichungsclivage' accompanied in some cases with a
certain amount of crushing along the transverse shearing-planes. See further
paper by the author read before Sec. B. (Brit. Assoc., 1888) and published
in extenso in the Chemical News, No. 1505.

NOTE M. (cf. p. 59.)
fossil- evidence of Extension in direction of Cleavage-dip.

The observation that fossils undergo frequent distortion in this direction,
and thus afford direct evidence of it in the genesis of a slate, is a very old
one. Much has been made lately of the case cited by Heim (see Marr, Brit.
Assoc., Manchester, 1887, reported in Nature, loc. cit. Note II.) of Belemnites
in the Jurassic slates of Switzerland being parted asunder transversely and
the zones interspaced with crystalline calcite. But facts of this nature are
not new.

In the Museum of the University of Zurich, as Gotta informs us in a letter
from that city dated 1849, there had been collected even then a numerous
suite of Belemnites from the Lias-formation, which for the most part had
been so altered by squashing (Quetschung) that they could only be recognized
as Belemnites by comparison of many examples. These Cephalopods (V.C.
goes on to say) " lie in a dark clay-slaty rock, and are for the most part, as it
appears, torn assunder (zerissen) into single parts through the squeezing and
stretching of the rock in the direction of its cleavage (Schieferrichtung), or
they have been stretched out into knotty staves which have now only a faint
resemblance to Belemnites. The interspaces (Zwischenraume) in the former
cases have been filled with slate." (cf. p. 59 of this work, also Note T. infra).

NOTE N. (cf. p. 78).

Von Cotta is precise in his account of the observations which he made of
the serpentinization of granite. He must tell his own tale :

" At Predazzo we happened for the first time to examine one of the places
where the limestone and granite are in contact. We selected the rock-wall of
the Canzacolli. Not without fatigue we climbed up on the boundary between

120 ROCK-METAMOBPHISM.

the granite and the limestone as far as the great quarry, in which beautiful
white marble is obtained, and then still further upwards to where the lime-
stone spreads out over the granite. As far as the marble-quarry we found the
boundary everywhere clearly defined and quite sharp. In many instances the
granite branched out in vein-like (gangformig) processes into the limestone,
and, (what is especially noteworthy) these at first (at their origin from the
massif) undoubtedly granite veins become, as they penetrate further into the
limestone, more and more talcose, and very soon pass over into undoubted
serpentine-veins, by which the marble moreover is often sharply intersected,
as seen in large fragments. The serpentine-veins had been previously observed
by Fuchs and Petzholdt ; but that they spring [as apophyses] out of the
granite, and still consist in part of granite, had no one, so far as I know,
before noticed." ("Die Alpen" : Weigel, Leipzig, 1881, pp. 196-197).

To leave nothing wanting in the precision of his statement, V. Cotta
figures the exact position of the spot where the observation was made, and
adds a coloured drawing to shew the exact relation of the serpentine both to
the granite and the marble.

NOTE 0. (cf. pp. 65-68.)
The Moon's Surface.

Does not Prof. R. S. Ball's theory as to the origin of the Moon suggest an
explanation of the result to which Zollner's investigations have led, that the
higher parts of the Moon's surface (those which appear to us illuminated) are
composed of materials which on the Earth would be regarded as among the
whitest of substances ? Can we not follow rationally the Moon in its earlier
orbits revolving round the Earth for some time within the limits of the
terrestrial atmosphere, then far more extensive than now, if we may judge from
the proportionate extent of the present atmosphere of the Sun ? And does it
not seem highly probable (by all physical considerations) that in the outer
perisphere steam and CC>2 (and perhaps other acid gases) would be much more
abundant than in the lower strata of the Earth's atmosphere, while the surface
of the globe itself was still in a glowing liquid state ? And would not the com-
paratively rapid cooling of the smaller lunar mass admit of condensation of
carbonated water from the outer terrestrial atmosphere upon the surface of
the Moon at an earlier period than that was possible on the surface of the
Earth to any very general extent ? Have we not here the factors needed for
converting the silicates at the surface of the lunar mass into carbonates and
free silica to furnish a white rocky outer crust to the JMoon ? In the absence
of a permanent lunar atmosphere and of permanent lunar waters, owing to the
greater attractive power of the Earth's mass, and the consequent re-evaporation
of any previously condensed free water on the Moon's surface, as that orb
passed beyond the limits of the Earth's atmosphere, would not such an early
crust remain intact (except where broken through by igneous outbursts) from
the absence of disintegrating agents?

Tidal waves. Of course, while the Moon remained unconsolidated earth-
tides must have been produced in it as the counterpart of the 'lunar tides'
produced in the Earth ; and these would be so much the greater in proportion
to the greater mass of the Earth at any given distance. May not such tides
in the Moon in its earlier individual existence along with the more rapid cooling
of the smaller lunar mass, explain in part the prodigious size of the mountains
as compared with the mass of the Moon itself? May not the 'rills' and 'walled
plains' be in such way connected with the earlier 'earth-tides' in the Moon?
Again, the appearances presented by such huge lunar volcanoes as Copernicus
and some others do not suggest the general radial arrangement of volcanic
outflows from a centre (as in terrestrial volcanoes) but a certain rough
parallelism in the ridges which bound this and the adjacent craters, as if the
craters were but eruptive openings through the crests of great tidal waves in
the partly consolidated lunar crust,

APPENDIX ii. 121

Perhaps the surface of the moon teaches us more than anything else what
the earth's surface was like in the earlier stages of the formation of a solid
rugged glowing outer crust, at a time when oceanic waters had not as yet
begun to condense upon it. It has been pointed out already that the
atmosphere of the Earth was then probably much more extensive, and that in
the outer regions of it the earliest water (steam), CO 2 , and perhaps other
gaseous oxides would be formed. The action of these upon the surface-
materials of the moon has been also suggested as probable; the only
assumption (and that not an extravagant one) being that the Moon's surface
advanced to some extent in the process of solidification while as yet within the
limits of the Earth's atmosphere. If this were so, it is almost certain that
much steam would be included mechanically in those slaggy portions of the
Moon's surface which solidified first, giving them a more or less pumiceous
character. The consequence of this would be that these solidified portions
with their included steam would be specifically lighter than the lunar magma
in which steam was not in this way imprisoned. They would therefore float
on the magma, just as pumice floats on water when its cavities are air-charged.
Now, if we extend the idea of solidification setting in at the crests of tidal
waves of the Moon's magma (which on physical grounds seems highly
probable not merely for the reason suggested in this work, but from the fact
that the mass of a wave would present a larger surface relatively to its mass
for loss of heat both by radiation and by convection-currents in the adjacent
terrestrial atmosphere,) and consider at the same time how waves of the sea
become charged with air at their summits so as to be converted into foam, we
perhaps have the clue to the striking and peculiar features presented to us by
some of the lunar mountain-ranges, especially that of the Lunar Apennines
(see Nasmyth and Carpenter's work, The Moon, Plate ix). On Plate xi of the
same work is shown a series of mountain-ridges, having such a marked
parallelism with one another as to be strikingly suggestive of a series of
'waves and troughs.' In the central one is the great lunar volcano
Triesnecker. In the broad valley on the left of this in the photograph is a
series of fissures ('chasms'), in some instances nearly a mile wide and 100
miles long. May not these, running, as they are seen to do, approximately
parallel to the mountain-ridges, be a series of huge shrinkage-cracks, owing to
the adhesion of the material as it solidified to the parts already solidified, thus
repeating essentially the phenomena of joints, of columnar structure, and
septaria? In the lunar region about Aristarchus and Herodotus the same
'wave-and-trough' structure is so remarkably well seen that the general
parallelism of the ridges seemed to the authors of the work to call for special
remark (p. 84). Fissures with a similar relation to the mountains are shown
here and in the broad valley by the side of the Lunar Apennines in Plate ix.
Other instances of 'wave-and-trough' parallelism (though less marked) are
shown in the frontispiece of the same work (Gassendi), in Copernicus (Plate
viii), in Mercator and Campanus (Plate xv) ; while in Plates x and xii a
certain general linear arrangement of the volcanoes, such as Judd's papers on
the Lipari Islands have taught us to refer to lines of weakness in the Earth's
crust, may be observed.

Taking the mass of the Moon as ^j-th that of the earth, it is manifest that,
at the distance of the Moon from the Earth at which the rigid crust began to
form on the former, the tidal waves would be much higher on the Moon than
on the earth; and owing to the greater mass of the Earth the conditions
required for incipient solidification would be reached in the process of cooling
by radiation much later on the Earth than on the Moon ; so that by the time
incipient solidification at the surface of the terrestrial magma became possible,
the Moon's distance from the Earth would have so far increased as to lead to
a very considerable lessening of the degree of tidal action produced by its
attraction on the terrestrial magma. From these two considerations we
should expect that, if such phenomena as are presented by the Moon's surface
repeated themselves at a later stage at the surface of the Earth, the altitude

122 BOCK-METAMOEPHISM.

of the ridges of elevation thus formed would be so much less on the Earth
than on the Moon that they might have been completely obliterated by the
action of subsequent oceanic tides. The argument for the flotation of the first
solidified portions upon the liquid magma on account of the inclusion of steam
applies to the case of the Earth as well as to that of the Moon.

NOTE P.

The case of the North- Western Highlands of Scotland.

In this work a suggestion is thrown out as to the possibility of tidal waves
initiating some of the lines of upheaval of the Earth's crust. In a somewhat
qualified form there appears to be some sense in that idea. We can conceive
the early formation of a solid thin crust corresponding to Zollner's third phase
of developement. Now for a long time this must have been so thin that the
tidal waves formed by the Moon's attraction on the enclosed magma must have
caused frequent ruptures of this crust, with eruption of portions of the enclosed
magma. This could scarcely happen without inducing foliation in the
intrusive masses, from the shearing produced by the sliding movement of the
solid portions of the outer crust, between which they were intruded ; but
what right have we to speak of this as even 'pre-Cambrian metamorphism,'
unless we so qualify the phrase that the term 'metamorphism' becomes
meaningless in so far as any definite sense has been hitherto attached to it in
geological literature? This point receives illustration from the remarkable
facts which the Director- General of the Geological Survey of Great Britain
laid before the Geological Society of London on April 25, 1888, (see Abstract
of Proc. No. 522). He describes, as occurring in archsean times in the N. W.
Highlands of Scotland,

(1) eruption of basic igneous rocks;

(2) the developement of a rude foliation in these masses probably by

mechanical means, and their arrangement in gentle synclines an<J
anticlines ;

(3) injection of igneous dykes into the original gneisses, consisting of

(a) basalts ( ? dolerites),
(6) peridotites and picrites

(d) granites ;

(4) mechanical movements giving rise to lines of disruption ; and producing

(5) changes of the more basic igneous rocks into schists, and of the granites

into gneiss.

Surely this looks like the kind of work which we should expect tidal waves
in the magma to do in the earlier stages of the formation of a solid external
crust, as I have suggested above. The penetration of the fissures and junction-
planes by such water as was locally condensed under pressure would give all
the conditions required for such paramorphic changes as are indicated.

The remark, that "there is overwhelming evidence to prove that all these
various changes had been superinduced on the archsean rocks in pre-cambrian
time/' is of the highest value as testimony from such an authority ; adding as
it does one more striking and incontestable instance in confirmation of the
generalization of Credner's quoted in this work (p. 17) "that the process of
metamorphism so-called, which distinctly characterizes the pre-Silurian (pre-
Cambrian) formations, was at the entry of the earth into the Silurian
(Cambrian) period already completed."

We have in fact one more chapter added to the record of the kind of work
which was going on in the developement of the present Earth during Zolluer's
4th phase of developement, the pre-oceanic stage of eruptive energy.

Evidence is brought forward of 'regional metamorphism' accompanying at a
later period the great terrestrial displacements which occurred in the mountain -
building stage, such as

(i) formation of new divisional planes in the Cambrian gneiss;
(ii) conversion of grits and shales into 'schists' ;

APPENDIX ii. 123

(iii) conversion of Silurian quartzites into 'quartz-schists' ;
(iv) crystallization of limestones ;
(v) foliation produced in igneous rocks.

On this the question may fairly be asked if the schists (ii) are really anything
more than phyllites, implying at the most mere 'cleavage-foliation;' also if
the production of 'quartz-schists' is intended to indicate anything more.

Kalkowsky describes such features of 'Quartzitschiefer.' "Viele Quartzite
sind deutlich und zum Theil dlinnschiefrig, und werden deshalb Quartzitschiefer
genannt. Diese vermogen auch transversale Schieferung anzunehmen, wodurch
sie bisweilen in zweischneidige, messerklingenformige Griff el zerf alien."
(Lithologie p. 27 QJ.

Upon the whole (with the exception of the dubious case of the 'schists' (ii),
which evidently need more definition, little, if anything in the way of para-
morphic change seems to be implied in the facts brought forward. Such
changes as have occurred in this later mountain-building stage appear to be
merely of a metatropic and metataxic character (mainly the latter). What
the distinguished author may mean by the "chemical effects of mechanical
movement" requires further explanation than is given in the abstract. From
the information before us they appear to be little, if anything, more than
physical and mechanical changes.

A perusal of the paper* in detail leads to such reflections as the following,

(1) What are these and similar 'eruptive dykes' of basic igneous rock of
archaean age ? Are they ' anogenic ' ? or are they ' katogenic ' ? Intrusivef
they undoubtedly are. But are they composed of highly basic materials
formed by reactions of the heavy metals (iron, &c.) of the bary sphere or
metallic kernel upon the earliest silica and silicates deposited, and squeezed up
from below through planes of weakness by the force produced by the ever-
increasing weight of the growing lithosphere ? Or are they on the other hand
infillings from above of huge divisional planes of contraction of the yet
glowing but partly-consolidated granitoid and gneissic masses, by such more

* See Q.J.G.S., vol. xliv, August, 1888, "First Report of the Recent Work
of the Geological Survey in the North- West Highlands of Scotland." On
reading the paper I ventured to write the criticism which here follows. This
I intended to read before Section C of the Brit. Assoc., but it was rejected by
the organizing Committee of that section on the ground that it was " too much
of the nature of a review," and that the section "could not judge of it
without the original paper before them." This of course was reasonable ; and
I have not thought it worth while to enquire into the constitution of the
' Organizing Committee.' Having been favoured however, while at Bath,
with a copy of the ' Essays on the Crystalline Schists ' contributed by eminent
foreign geologists for the International Congress which met in London on
September 17th, 1888, I was struck with the emphatic way in which Prof.
Heim's remarks in the concluding paragraphs of his Essay gave support to my
contention ; so much so, that on the 16th of September, I sent the paper off,
just as it was first written, to the Editor of the Geological Magazine. He
kept it for two months, and then returned it to me stating in usual editorial
phraseology that " want of space had prevented his being able to publish it."
This statement I have found difficult to harmonize with his embarras de
richesse in the matter of space some three months previously. Perhaps the
audacity of this criticism took Dr. W's breath away, or he generously thought
to save me from the discomfort of having to confront the awe-inspiring
visage of the Sphinx of Jermyn Street. Strengthened in my conclusions by
an examination of the rock-specimens exhibited last September, I publish it
here in the form in which it was originally written.

f ' Intrusive ' (that is to say) as distinguished from segregative. I do not
believe in the pretty hypothesis of ' segregation-planes,' which a few geologists
have evolved out of their imagination.

124 ROCK-METAMOEPHISM.

basic materials as are found everywhere, where the normal developement can
be studied, in the overlying archsean crystalline schists ? We look with the
keenest interest for evidence to guide us in the choice of these alternatives .
At present not a fragment has been furnished.

Our thanks are due to the authors of the paper for the thoroughness of
their observations on the igneous intrusive rocks of the area dealt with ; a
thoroughness such that it has added one more region of the earth's surface to
those in which observations of a similar kind had been previously made.
The conclusions of Suess* from observations made in the whole region of the
Austrian Alps and the Carpathians, as to the 'passivity' of igneous intrusive
rocks in the process of mountain-building, were amply confirmed ten years
ago by Heimf for the Central Alps, for the intrusive porphyries, mela-
phyres, granites and syenites alike ; and now the classical region of the N.W.
Highlands has been made to bear testimony to the far-reaching truth of
Suess's induction.

(2) Reference is here made to the passage " Reflecting on these facts ....
originally deposited." (See Note H. in this appendix.)

These general remarks apply I take it with some modifications, to the
'metamorphosis' of the Cambrian and Lower Silurian strata of the N.W.
Highlands of Scotland so powerfully described in this paper. The splendid
work of Messrs. Peach and Home and their colleagues has brought to light
the extreme results which pressure-movements locally intensified are capable of
producing. The most elementary principles of mechanics however are enough
to show that such intensification of mechanical force and its transformation to
a large extent into heat must be local. The less-yielding character of the
rocks here than in those of the Todi-Windgallen group has given us a
predominance of crush-planes of thrust rather than of folding without
fracture ; otherwise the phenomena described seem to be essentially the same
as those previously described by Heim ; and they add nothing to the
possibilities of which we were previously cognizant. Such local effects of
mechanical work cannot furnish a basis for a general theory of ' regional-meta-
morphism ' applicable to the 30,000 metres of gneiss and schists of archcean age,
exposed to the light of day over large areas of the earth's present surface, and
probably extending under the sedimentary formations round the whole sphere
of the Earth. The author of the paper will have to pardon me therefore for
saying that " It is [by no means'] obvious [to those who look at the matter with
a more critical eye] that the facts brought forward furnish a large amount of
evidence in support of the theory that regional metamorphism is due to the
dynamical effects of mechanical movement acting alike on crystalline and clastic
rocks." $

(3) One broad result of their investigation is the contrast throughout the
area dealt with, between the mechanical metamorphism of the Cambrian and
Lower Silurian, and that primary morphology which characterises the Archaean
rocks of this small area in common with that which they display wherever
they have been examined in all the four quarters of the globe. Further, until a
thorough microscopic examination is made, the above conclusion is at least
premature. Better would it have been for science to have imitated the caution
of Heim. (op. cit. Bd. II, p. 40, et passim.)

In the similar case of the Central Alps (ibid, loc. cit.), " A very essential
difference exists in their entire facies between the rocks of the central-massif
and the younger sediments. In those schistosity (Schieferung) predominates,
in these stratification (Schichtung) ; in those, steep, even, and regular position
of the structural planes (Schiefer), in these, flexures, and even layers ; those
are formed for the most part of quartz and silicates, these of limestone and
clastic materials (Triimmergesteine). The two different rock-groups can be

* Entstehung der Alpen, Abschnitt, I.

t Mech. der. Oebirgsbildung, Bd. II., p. 124,

% Q.J.G.S., (loc. cit.) page '438.

APPENDIX ii. 125

distinguished at a distance by their different forms resulting from weathering
and denudation (Verwitterungsgestalten). The rocks of the central-massif
have yet to be proved to be aqueous sediments ; many of them belong probably
to the earliest lithosphere (Erstarrungskruste) of the Earth."

A perusal of the paper above referred to, (which has been hailed as an
epoch-marking production in certain quarters)* leaves upon one's mind the
impression that, with some modifications of detail, the above contrast holds
good for those fragmentary remains of a great mountain -chain in the N.W. of
Scotland.

(4) When we bear in mind that the contraction of the outer crust as the
kernel became in course of time too small for it, on account of

(a) primarily, contraction due to continuous cooling and solidification ;

(b) secondarily, the loss of material from the interior through volcanic

eruptions

has given us (altogether independently of any theoretical views) the' following
stubborn factsf :

(i) that the volume-proportion of the mountain- chains of the globe to the

continents is such that the mountain-chains are quite subordinate

phenomena ;
(ii) that the continental masses rise above the ocean of the present globe

as huge broad pedestals upon which the mountain-chains rise as

little ribs, adding about -g-^-Q to their volume ;
(iii) that the movements of the Earth's rind which have differentiated the

continents and the oceans have been different from those which

have produced these mere wrinkles upon the great continental

plateaux, though the force is primarily the same ;

we find it by no means possible to proceed so jauntily as some of our
brethren of the hammer seem capable of doing, from the contemplation of
such local, intensified, and extremely variable results of intense localization of
mechanical force in the process of mountain-building, to the inference that the
agency which has given us these special local results, in ^-Jo" part of the
elevated regions of the earth, has caused also the prevalent morphology of
those vast regions of the remaining ^ij-jj, which still remain intact and proof
against the agencies of denudation and degradation, or to leap such a mental
' Niagara ' as is implied in any application of such results to the genesis of the
great Archaean crystalline series of rocks.

(5) What are these huge ' thrust -planes ' but planes of comparative weak-
ness in the outer crust of the globe ? They surely represent in the North-
Western Scottish area the great overfolds of the Central Alps ; the results in
both cases of the resolution of normal into tangential thrust, due to the
contraction of the Earth's mass. Their magnitude strikes the mind of the
observer with an astonishment, which under its first spell is almost
bewildering ; yet they are but trifles when viewed as part of the whole sphere
of the Earth. Take the case of "the disrupted rocks which must have
travelled forward several miles from the east." ("Report, &c.," p. 429). A
comparison of the sectional diagrams (figures 9-21) justifies us, I think, in
regarding five miles as a liberal interpretation of this statement. If we allow
25,000 miles for the circumference of the Earth in palaeozoic time we get

5 miles (measured circumferentially) = ^-girnu = 5 (fo Q"
of the Earth's circumference. This would represent a radial contraction =

27T

rather more than -5 o o~o or little more than an 800th of the Earth's radius.
Here however we are dealing with a maximum of circumferential movement.
Had we the data for reducing this to the mean circumferential movement
round a great circle of the globe, we should no doubt have to multiply our

* See the discussion on the 'Report,' Q.J.G-.S., loc. cit.
t Heim (op. cit.) Bd. II, p. 236, et. seq.)

126 BOCK- METAMORPHISM.

denominator by a factor of some hundreds. This rough calculation is put
forward as a caution against hasty inferences from localized results of locally-
intensified movements, as to the work done on a terrestrial scale.

In the slow contraction of the terrestrial sphere circumferential approxima-
tion of the more stable portions of the crust must come about. This will be
admitted by everyone as an absolute necessity. What is to become of the
intervening and less stable portions ?

If we confine our attention to such a portion as has evidently been caught
in the grip of two such massifs as the great Bohemian core of the European
continent and the central massif of the Alps, we get a pretty clear idea of
the main result, in the excessive over-folding of the outer flanking strata of
the Eastern Alps, as long ago pointed out by Suess. A similar explanation I
believe applies to the excessive folding of the Central Alps, when we view the
position of these great disturbances with reference to the great massif of
which the Schwarzwald and the Vosges are indications ; while between these,
and vis-a-vis of the great plain of Bavaria, there is no such excessive folding
of the outer flanking strata of the Alps on the north side.

Substitute now for these comparatively soft and yielding strata, with their
'latent plasticity' (Heim) allowing distribution of the energy of movement
more or less through ths mass the more fragile materials of the Cambrian
and Lower Silurian strata of the N.W. Highlands (as evidenced by the results
observed) ; and we get thrust-planes taking the place of over-folding (in the
main), with a consequent concentration of mechanical work along these planes,
and as a further consequence intensification of the heat-energy into which the
mechanical force is transformed. It is not unlikely indeed, from the general
conditions of the problem, it is likely that the movements would be more
rapid in the case of the displacements in the N.W. Highlands than in the
more overfolded strata of the Alps ; and this of course would intensify (cet.
par.) the heat-energy developed. Exceptional conditions brought about in
this way must give us exceptional results. Look at it how you will, in the
light of broad general principles, you cannot logically construct a theory of
'general regional metamorphism^ out of suck exceptional data obtained from a
few circumscribed areas.

(6) Lastly, a general unsatisfactory impression is left upon the mind that
the scientific value of this first outline "Report" is much deteriorated by the
vague indefiniteness with which the technical terms 'gneiss,' 'schist,' and
' slate ' are used. Each of these words appears to be used in somewhat
different senses in different parts of the paper. This much regrettable feature
of it could have been avoided, and the scientific value of the production
greatly enhanced, by the employment of a dual terminology, such as is so
commonly in use among the leading continental geologists.

So far however from any desire to depreciate the value of the work of
Messrs. Peach and Home and their colleagues, I say again, that the scientific
world owes them a great debt of gratitude ; for I am convinced that the facts
rightly interpreted will be found to have ' driven another nail into the
coffin ' of what Prof. Credner, in a letter to me lately, has characterised as
fanciful ideas (' phantastische Ideen ') on 'general and regional metamorphism.'

NOTE Q. (cf. p. 80.)

The formation of minerals of the Spinell family in metamorphosed lime-
stones at a dry heat has received illustration from the observations of Stelzner
and Schulze at Freiberg (Credner, Elem. der Oeol. 6th ed. p. 313). The walls
of the clay muffels used in the reduction of ZnO were found converted into a
mass of zinc-spinell and tridymite, although these walls had remained solid
(but porous) at a temperature of 1300C for a week together. The zinc
would appear to have penetrated the walls in the form of vapour, along with
(probably) dry steam. This seems to point to a process of the nature of
'fritting' in the synthesis of the spinell, the Alj Oa being taken up at high

APPENDIX ii. 127

temperature by the oxides of zinc, iron, (and magnesium.) We can understand
the formation of Zn O by the reducing action of the hot zinc-vapour both
upon Fe 2 0$ (of the clay) and Hj 0, thus

Zn O + 2Fe O;

With the latter we are familiar.

Then, by a fritting process, the A1 2 Og would be taken up by the ZnO, FeO,
(and Mg O, if present) from the silicates of alumina contained in the clay, the
Si Oz being set free, and crystallizing as tridymite. In such a case H 2 O
could clearly be present only as a dry gas, and laboratory- experience leads us
to suspect that the spinells were all formed in the dry way at high
temperatures, since the soluble aluminates are very unstable in the presence of
carbonated water. (Koscoe and Schorlemmer, op. cit., vol. ii, part i, p. 443).
I have observed that such an aluminate as that of potassium (which is formed
when hydrogen is made by the action of aluminium upon potassium hydrate)
undergoes decomposition in a day or two, an amorphous earthy powder of
alumina being precipitated. The 'earthy dust' of authors may result in
some cases in the archsean rocks from the action of water upon aluminates of
the alkalies (and perhaps also of the alkaline earths) which were first formed
along with the insoluble aluminates of less strongly-positive bases (the
spinells). It is conceivable that this may have been a pretty fruitful source of
free alumina (first produced by direct oxidation of aluminium), in the form of
amorphous oxide or hydroxide, or as subsequently taken up by Si 2 to form
silicate of alumina under suitable conditions. The facts seem to lend them-
selves to the idea that a considerable proportion of the alumina of the crust of
the earth may have been in the first instance appropriated in the dry way by
the bases of the alkalies, as aluminates, and that the nascent Al 2 Og furnished
by the decomposition of these aluminates was thus in a condition specially
favourable for combination with Si O 2 .

It is an interesting point to note here (cf. p. 12 of this work) that on
melting

but =00, ., 719);

from which it appears that at a glowing heat A^Oa, like SiO 2 , can expel one
molecule of C02 from the alkaline carbonates, uniting with the base to form
an aluminate, as SiOa does to form a silicate.

NOTE R. (cf. p. 96).
Fahlbdnder.

The possible origin of these suggested in this work removes the necessity
for regarding the formation of the great beds of iron-ore found in them as
due to the intervention of the action of organic matter, and their presence
therefore as affording evidence of the existence of vegetation in archaean time
(see Sterry Hunt, Chem. and Oeol. Essays, pp. 13, 301, quoted by Etheridge
in his Presidential Address to the Geological Society in 1881). The fallacy is
repeated by Geikie ('Text-book,' p. 639, ed. 1882). The direct product of
combustion of iron- vapour is Fe 2 O 3 . This is a matter of direct observation
when a steel watch-spring is burned in a jar of oxygen. There are two
oxides formed, Fe 3 64 which falls down in molten masses, and Fe 2 O 3 which is
given off by the flame (i.e. the burning vapour, the iron being partly
volatilized by the intense heat of combustion, as the wax is in an ordinary
candle-flame), the latter condensing as a brown sublimate on the walls of the
jar. Three of the most noteworthy facts in connexion with the Fahlbander
are (1) the quantity of free Si 2 often present as quartz, (2) the abundance of
magnetite, (3) their frequent inter-bedded relation with hornblendic schists.
(1) and (3) seem to suggest the deficiency of good alkaline fluxing materials to
promote combination of the oxides of iron and the silica by fusion, while the
magnetitic form of the iron-oxide suggests the action of iron-vapour at a

128 BOCK-METAMOBPHISM.

high temperature upon steam, as in a well-known laboratory-process. On the
other hand it is possible that some of the phenomena presented by the
Fahlbander might be explained by the hypothesis of local rapid cooling by the
down-rush of great masses of cooler air and vapours somewhat as appears to
happen in connexion with 'sun-spots,' and compared with which the greatest
atmospheric disturbances within the range of human experience are probably
mere trifles.

NOTE S. (cf. p. 92.)
Relation of Organic Developement to Physical Environment.

A comparison of Mr. Etheridge's analytical tables of palaeozoic organic
remains in his Address to the Geological Society in 1881, leads to one or two
conclusions of great theoretical interest. We note the absence throughout the
earlier palaeozoic formations of (1) plant-remains (with a few doubtful
exceptions,) (2) air-breathing animals, (3) fishes. May it not be that the
atmospheric conditions were as yet unsuitable for the existence of such forms
owing to the presence of a large excess of C0 2 in the atmosphere (and
therefore in solution in the waters of the ocean) and a corresponding
deficiency of oxygen? The appearance of fishes in the Ludlow period, and the
rapid increase of that class in the Devonian seems to suggest that at about the
close of the Silurian period those conditions were being gradually reversed,
until in the Carboniferous period the quantitative composition of the
atmosphere (and consequently the aeration of oceanic waters) permitted the
first appearance of Amphibia along with 290 species of fishes and 339 species
of plants (mostly vascular cryptogams). The plentiful supply of the food-stuff
of plants (C0 2 ) as compared with the proportion of that gas present in the
atmosphere of later periods (though no longer in such excess as to interfere
with their vitality) may well explain the enormously rapid assimilation of
carbon (as has been suggested by Prestwich) which resulted in the formation
of the Coal ;* and obviously such an enormous developement of plant-life, as
the great agency for the storage of carbon in the earth's crust, w r ould tend to a
rapid purification of the atmosphere, so as to render it fit for the support of
air-breathing forms, which appear to have increased rapidly in mesozoic time.
The absence, of land-plants in the earlier palaeozoic periods might be perhaps
accounted for by the absence of dry land; but we cannot account for the
absence of fishes in the same way. Dr. Selwyn (whom I had the pleasure to
meet at Birmingham when the British Association met there in 1886) stated
to me in conversation that he thought it likely that there was no very general
elevation of dry land before the Devonian period, though I do not recollect
what the data were on which he based that view. But it seems to be
supported by the facts just referred to, so far as negative evidence can go to
prove anything. If however we recollect what has been urged of late as to
the great magnitude of the earlier oceanic tides, we see that in earlier
palaeozoic time any elevated region would have to be many times higher above
mean sea-level than was required in later and recent times in order to form
dry land. It may be therefore that though such great archaean land-surfaces
as that of North America have never been permanently submerged as a whole
since their first elevation in later archsean time, yet that elevation (being
regional)f was probably very gradual and permitted the rolling of great tides
over them for ages, with the consequent proportionally greater scour and
mechanical disintegration of the rocks. In this way some light seems thrown
upon the history of the Cambrian conglomerates.

* See also note by the author on ' The Relation of the Percentage of COa
in the atmosphere to the Life and Growth of Plants.' (Brit. Assoc. Report,
1888, p. 661).

t See Credner, EUm. der Geol. pp. 189, 90 ; also Suess, Entstehung der Alpen last chapter
' Antlitz (features) der Erde,' and Heim, Mech. der Gebirgsbildung.

APPENDIX ii. 129

NOTE T.

The Airolo Series.

It may be well to consider here a thorough-going investigation of a
coinplexus of shales, quartzites, and impure limestones, which Dr. Ulrich
Grubenmann of Frauenfeld has recently made. * The series has been
(apparently) enfolded in a sharp syncline between the two enormous gneissic
massifs of the St. Gotthard on the north, and the Tessina group on the south.

We know on pretty sure evidence that the series of movements from
which the present Alpine system has resulted, began (at the latest) before the
end of palaeozoic period. Such movements could hardly take place without a
certain degree of flexure in the archsean gneisses and schists. Some of these
synclinal flexures would be for a long time below sea level, as the Airolo-
syncline must have been, if these later sediments (as the Swiss geologists
believe) are of marine origin and of the ages of the Trias and the Jura.
These sediments have undoubtedly undergone in the later mountain-building
stages, an enormous squeezing and squashing, as they have been folded back
upon themselves, so as to compress them (according to Grubenmann) into less
than ^th of their original dimensions, the synclinal flexure having been
accentuated into a sharp synclinal fold ('Mulde'). Here we find such
distinctly individualized minerals as garnet, of which however no analysis is
givenf; disthene (Al 2 SiOs) with its variety kyanite; two micas, margarite (a
lime-mica approximating in composition to the formula AUCaHoSigOis), and
meroxene (a magnesia-mica, a variety of biotite, of a much more complicated
structure than the former) ; quartz in grains and layers ; tourmaline in small
needles ; rutile ; zoisite in small prisms ; zirkon (occasionally met with) in
rounded elongated grains ; magnetite ; calcite, often so intimately associated
with quartz as to point to the reaction of COa upon CaSiOs as its probable
origin ; all these occurring in a series of schists intercalated with quartzites,
where the field evidence shows that the rocks have been subjected to a
maximum of pressure.

Are we bound to follow Grubenmann and others in attributing all these
results to ' dynamo -metamorphism ' ? Hardly ; and that for the following
reasons :

(1). As to the age of the sediments. They are assigned to the age of the
Trias and the Jura because the lower strata contain much gypsum and
dolomite, and in the garnet-bearing schists, belemnites are said (on the
authority of Studer) to occur at Fontana, and on the Nufenen Pass between the
Val Bedretta and the Upper Rhone Valley, while in rocks in Val Piora, which
are of similar nature to the ' Kalkglimmerschiefer ' (which constitutes by far
the largest mass of rocks entrapped in this synclinal fold), it is said that
Escher V. d. Linth found ' undeterminable but undoubted organic remains '
(unbestimmbare aber unzweifelhafte organische Reste). The first of these
facts, the occurrence of gypsum and dolomite, can hardly count for much in
determining the age of the rocks ; if the belemnites are not better preserved
than those said to occur in similar schists in specimens exhibited by Prof.
Heim at the International Congress, they can afford at best but dubious
evidence ; and, lastly, the collapse of the theories which have been built upon
the supposed organic origin of the structure known as Eozoon Canadense

* ' Ueber die Gesteine der sedimentaren Mulde von Airolo ' ; Mitth. der
Thurg. Naturf. Gesellschaft, Heft viii, (Frauenfeld ; Hubers, 1888.)

t This is much to be regretted, since a comparison of their composition with
those of neighbouring archaean crystallines would have been of great value in
the investigation.
K

130 EOCK-METAMOKPHISM.

ought to be a warning against attaching any serious importance to supposed
' undeterminable organic remains.' But apart from the question as to the age
of the sediments, which is but a matter of secondary importance, we must
consider

(2). The far more important question as to whether the garnets, disthenes
and micas are authigenous or allothigenous. Grubenmann seems to assume the
former to be case. Yet there are facts stated in his paper which should make
us hesitate to adopt this view. For the garnets are in many cases flattened by
pressure, and the appearance of the garnetiferous schists of a similar history
which Dr. Heim exhibited in London in 1888, seemed to me hardly recon-
cilable with an authigenous origin for the garnets at least ; so much so that I
made a suggestion to him to that effect. Dr. Grubenmann, moreover, tells us
that as the succession of the inverted schists is followed up the mountain-side
in the Airolo district, the highest zone (with two micas) " passes over into the
amphibole- and garnet- bearing schists of the southern schist- and gneiss-zone of
the Gotthard-massif ; * a fact which points strongly to the derivation of the
materials of these altered shales of the anti-clinal fold directly from the archcean
schists and gneiss, the two series having become welded together by the
pressure which has induced the ' schistosity ' observable in the sedimentary
series.t There is another strong fact against the authigenous origin of the
contained minerals ; that is to say, most of the minerals contained in the
'Schiefer' occur also enclosed in the gypsum, which is found near the base of the
series. Quartz, pyrite, mica, talk, tourmaline, disthene, and zirkon are
mentioned as occurring embedded in the gypsum (anhydrite), out of which
they could scarcely be fabricated. Disthene we know is a very stable mineral
and endures mechanical transport by water to almost any distance without
being decomposed, since it is found in "clear rolled crystals in the gold-
washings on the south side, of the Urals " (Naumann) ; and Mr. Dick has
recently found kyanite along with zirkons, rutiles, and tourmalines, in the
Bagshot Sands of Hampstead. (Nature, vol. xxxvi, p. 91). Teall has also
recently described rutile as occurring in some clays. Quartz, rutile, tour-
maline, iron oxide, pyrites and calcite are constituents of Thonschiefer.J
In the case before us the orientation of the micas, the disthene, and the rutile
points to their pre-existence in shales, whose lamination planes have initiated
the ' schistosity ' subsequently induced by pressure. (Supra, pp. 60, 61.}

(3). If we admitted that some of these minerals are authigenous, and that
pressure had given them in some cases their crystalline form (an essential
factor certainly to their individuality qua minerals), it by no means
follows that their present chemical composition is the result of molecular
changes, caused by the intense pressure which has induced a schistose structure
upon these sediments. The gypsum was pretty certainly precipitated as the
result of chemical reactions in the waters of this confined and narrow area, as
was also probably the dolomite (see pp. 9-10 of this work) ; a large proportion
of the carbonate of lime might have been also precipitated, resulting from
such reactions as are instanced on p. 6 of this work, which is the more
probable from the fact which we may fairly assume that most of the water
drained into the quondam area of sedimentation was received from regions
consisting of rocks more or less rich in felspar ; and the occurrence of CaCO 3
in some of these rocks intimately associated with Si0 2 seems to point to the

* It may be well to note here that Grubenmann seems to have no difficulty
in distinguishing these two series of ' Schiefer ' from one another.

t The neighbourhood of the great intrusion of granite of Pitz Rotondo and
Pitz Pisciora ought not to be overlooked.

J Kalkowsky (op. cit., pp. 257-8). The determination of the authigenous
or allothigenous origin (in rocks) of tourmaline is (he remarks) well-nigh
impossible, since these tourmalines occur with the same properties in the
phyllites and mica-schists as well as in sandstones, conglomerates, &c.

APPENDIX .

probable reaction of free C0 2 upon silicates containing CaO. As for the lime-
and magnesia-micas, it would be difficult to deny that they may have been
formed from muscovite, derived mechanically from the neighbouring archsean
rocks, by such reactions upon it of the salts contained in the sea-water, as are
suggested on p. 29 of this work, the richness of these waters in salts of CaO
and MgO being evidenced by the gypsum and the dolomitic rocks. Again, in
the decomposition of the felspar, which must have formed a portion of the
detritus of which these sediments are composed in order to furnish their
argillaceous materials (' Thon'), it is at least rational to suppose that some of
the silicate of alumina may have acquired a more basic character, such as is
indicated by the formula A^SiOg, which, as the analyses show, represents the
composition of the disthene here. On the other hand, the direct derivation of
disthene from the rather unstable mineral zoisite involves a comparatively
simple reaction in the wet way :

O 2 = 4CaC0 3 + H 2 -f 3Si0 2 + 3Al 2 Si0 5 *

Zoisite. Disthene.

That this is no vagary of the chemist is proved by Dr. Grubenmann's own
observation (op. cit. p. 15) of the passage (in the weathered rock) of zoisite
into CaCO 3 , Si0 2 , and (in some cases) disthene in the cracks and fissures of the
zoisite itself.

Substitute in the above reaction K 3 COs for free CO 2 , and we should get the
reaction for the conversion of zoisite into white potash-mica, which is a matter
of observation (Roth, op. cit., p. 352). If such changes, of a purely chemical
nature, took place in the fine silt derived from the adjacent siliceous rocks
antecedently to the subjection of the sedimentary mass to the pressure which has
given to these rocks their present schistosity, it is clear that we should have no
right to attribute them to the operation of that pressure ; even though we
admitted that this might operate upon materials of the right chemical
composition (previously formed by ordinary chemical reactions) so as to impart
to them a crystalline structure and form. (cf. pp. 54-55 of this work).

" It is easy [as Dr. Grubenmann sagaciously remarks] to be tempted to allow
too bold a flight to hypothesis in the region of rock-genesis " ; and our only
safe rule is to give hypothesis no place until we have exhausted all the
resources which the ever-active known principles of hydro-chemical action can
furnish towards the explanation of the facts observed. In the present instance
of extreme mechanical action within a very limited and confined space, we
have probably the ultimate results of the action of pressure upon materials
previously prepared for it ; the whole observed phenomena being not the result
of pressure merely, but rather the resultant of the combined factors of time,
hydrochemical change, physiographic conditions,! and pressure, with its
concomitant metataxis, and perhaps in some cases metatropy.

(4). On general grounds it may safely be asserted that conditions of high
temperature are those favourable to the building-up of the more complex
silicates ; while at lower temperatures and in the presence of water the
tendency is towards the resolution of these into simpler or proximate
constituents. The one is synthetic, the other is analytic. The vast difference
in the degrees of probability of these two processes is at once appreciated by
the chemist ; and the experience of the chemical laboratory is on this point in

* See Naumann-Zirkel's ' Mineralogie.' Graphically we might represent
the composition of disthene thus :

}(Si0 3 )
Al

the radicle (Si0 3 ) being the exact analogue of the (C0 3 ) of the carbonates.

f Anyone who has taken careful note of the nature of the detritus of
modern valleys in the gneiss and schist regions of the Alps will appreciate the
importance of this consideration.
K 2

132 BOCK-METAMORPHISM.

perfect accord with observations in the field and under the microscope, as the
study of secondary paramorphism soon reveals to us. The tendency of zoisite
to undergo decomposition (which is pointed out by Dr. Grubenmann), the
much greater abundance of disthene than of zoisite in these altered shales, the
decomposed state of the garnets* and of much of the zoisite which remains,
the way these two minerals (as also some of the others) are fractured and
water- worn, the frequent accompaniment of these garnets by such well-known
decomposition-products of garnet, as iron oxides, and quartz, (which commonly
separate out as the lime of a garnet is removed in solution and the iron gets
oxidised from the protoxide into the peroxide), and a comparison of these facts
with numerous instances which Justus Rothf has collected of the formation of
decomposition-products from garnets, the general occurrence of clay and
carbonaceous matter in the most highly-altered of these shales, the fractured
and worn condition of many of the quartz-grains, seem to furnish strong
cumulative evidence of a very considerable period of exposure of the most
characteristic minerals of these thin shales to conditions inducive of chemical
change, before the enormous pressure, which has given their present structural
character to these altered shales, quartzites, and limestones, was brought to
bear upon them. The absence of any record of felspar in these sediments is
significant, as pointing again to a vast period of hydro-chemical action upon
the original detrital minerals ; and the probability of this is heightened by the
single reflection that if the Swiss geologists are right in assigning these
altered shales to the age of the Trais we have in this Airolo district a
very insignificant series of shales and quartzites, not approaching it appears
anything like 50 metres in thickness, to be regarded as the equivalents in time
of the Triassic Dolomitic Series, which in the Eastern Alps is measured by
thousands of metres.

(5). The succession of the strata under consideration is by no means
certain, as Dr. Grubenmann candidly admits ; the field-evidence, owing partly
to alluvial deposits, and partly to the appropriation by a dense vegetation of
the rich soil furnished by the weathering of the rocks themselves, being
altogether insufficient for determining this with certainty. But this uncer-
tainty leaves the broad distinction between these localized rock-phenomena and
the great archsean series of gneisses and schists as clear as ever, except perhaps
to those who only know Alpine geology through the medium of books.

A glance at Studer and Escher's Geol. Map of Switzerland, is almost
enough to show one that the principal major longitudinal line of flexure in
the Central and Western Alps is that which commences with the valley of
Chamounii, (containing the Vernayaz series referred to above!), is continued
along the valley of the Upper Rhone (all that part above Martigny), the
Urseren Thai, and the valley of the V. Rhein down to Chur. On general
grounds, and taking into account the distribution of the 'Verrucano,' and
' T. Anthracit ' along this zone of the Alps, there would seem to be little room
for doubt that the initiation of this line of flexure dates from Palaeozoic time ;
nor am I aware of any facts in the geognostic structure of the Alps which
militate seriously against such a supposition. The protrusion of the great
granite-gneiss-schist massif of Pitz Rotondo and St. Gotthard, has caused a
bifurcation of this great line of flexure as we trace it eastwards, and has given
us, as a consequence, the line of flexure of the Val Bedretto and the Val
Leventina, which is deflected still more to the south by the great crystalline
massif of the mountains which furnish the head- waters of the H. Rhein

* So much so that the garnet could not be separated from the accompanying
minerals in sufficient purity for an analysis to be made.

t Allgem. und Chem. Geol., pp. 352-363. See in particular Lemberg's
experiments on Grossular (p. 363). J See page 90.

I am of course aware that Dr. Heim considers the architectonic structure
of the Alps to have played only a very insignificant role in determining lines of
erosion and denudation, but in this I am unable to follow even so great a master.

APPENDIX ii. 133

As narrow zones of sedimentation they must date from a very early stage in
the building of the Alpine system, and the nature of the sediments formed
here would differ (as pointed out above) presumably from those formed in
more extended basins, so as to afford special facilities for pressure to impart
to them such a ' schistosity ' as may well simulate true foliation. There is no
question here it appears of the grey schists (schistes gris = Biindner Schiefer)
which form a narrow zone all along the south flank of the Val Bedretto, but
only of a very limited series, their exposure in the section worked out by Dr.
Grubenmann, not amounting even when reduplicated by overfolding (in part)
to more than a very small fraction of the whole section, in which impure
limestones and quartzites predominate.

NOTE U.

On the Developement of the Archcean Crystallines. *

" The stratified sedimentary formations (palaeozoic and neozoic) form a great
series, whose origin and present developement we can for the most part explain
with apparent certainty. But besides these there exist at the base of our
formations vast series of strata of gneiss, mica-schists, phyllites, hornblende-
schists, chlorite -schists, talc-schists, and granulites, interstratified with which
occur granite-gneiss, marble, graphite, and iron-ores, as to the mode of
developement and building-up of which the views of geologists are still not
merely confused and indefinite, but to some extent widely diverse from one
another.f The originally sedimentary mode of developement of all this
complexus of archaic gneisses and schists cannot be doubted : in their strati-
form differentiation, in their massive structure, and in their parallelism, which
dominates the whole series, we are presented with a stratification (Schichtung)
as real as in the fossiliferous series of clay-slates, limestones, shales and
sandstones : concordantly with their distinctiveness as stratified masses
frequent interstratifications (often on a diminutive scale) of the most diverse
kinds of rock repeat themselves ; between gneisses and mica-schists regular
layers of granular quartzite and of conglomerates are met with ; with the
greatest regularity gneisses and granulites of the most manifold varieties are
interstratified with chlorite-, talc-, mica-, quartzite-, and hornblende-schists ;
between these may occur layers of granite-gneiss, beds of crystalline limestone,
magnetic iron-ore, and graphite, as well as complexes of graphite-schists ;
finally the gneisses pass into mica-schists, these into phyllites, and these again,
so far as the stratification-order indicates, into the lower palaeozoic fossiliferous
formations : the phenomena as a whole indicating some sedimentary mode of
developement of the gneisses and crystalline schists. Their present rock-
character is, however, according to the view of many geologists, not original,
but rather a change, a metamorphism has taken place in the course of time in
the originally clastic material, out of which have arisen the crystalline structure
and the petrographical habit which the gneisses and the crystalline schists at
present exhibit. This process of alteration has been termed 'general meta-
morphism ' or ' regional metamorphism.' As to the nature and origin of it the
views however differ widely. By some it is regarded as the result of the
active agency of the high temperature of the glowing internal mass of the
earth ; by others, as the result of intense pressure ; by others again, as the
result of hydrochemical processes, that is to say, of chemical processes carried
on through the agency of fresh water penetrating to depths. In other words
it is regarded, on the one side as a reaction of vulcanicity, on the other as
atmospheric in origin.

* Translation of 13 of the ' Petrogenetische Geologic' of Credner's
'Elements,' 6th edition, (Leipzig, 1887).

t J. Roth, * Ueber die Lehre von Metamorphismus aud die Entstehung der
krystallinischen Schiefer,' Berlin, 1874. G. W. Gumbel, 'Ostbayer. Grenzgeb.'
Gotha, 1868.

134 ROCK-METAMOEPHISM.

Plutonic Regional- Metamorphism.

"According to the views of the Huttonian school, and especially more
recently of Lyell and von Cotta, the metamorphism of the originally sedimen-
tary material is the result of a long-continued heating through the internal
heat of the Earth, whereby, under the influence of the simultaneously existing
pressure of the overlying formations, a melting of the lowermost rock-masses
concerned and an intimate recrystallization with transfer of material and
reconstruction have come about. In this process an important part is assigned
on the one hand to the waters which originally filled the interstices of the
sedimentary rocks, owing to its increased efficiency as a vehicle of heat by
convection and as an active agent in the solution and decomposition of
mineral-matter in the super-heated state ; on the other hand, gases and
vapours developed in the glowing-liquid kernel of the Earth and driven under
pressure through the rocks are considered important factors in metamorphism.
In plutonic regional metamorphism the processes would thus be similar to
those of contact-metamorphism in the neighbourhood of eruptive rocks, except
that they would arise from an universally-active source of heat, the glowing
interior of the Earth. Since now the metamorphic influence of the latter
diminishes in intensity from the interior towards the periphery, the most
deeply seated are by it affected and transformed in the highest degree. On
account of this the gneiss occupies the lowest position, and above this follow
in succession the mica-schists, the chlorite-, talc-, and hornblende-schists, and
lastly the phyllites so closely resembling the shales (Schieferthon). There
would follow however instead of a mere partial softening or recrystallization a
complete fusion of the originally sedimentary rock-material, so that this would,
like our volcanic lavas, be forced in the plastic condition into clefts and fissures
to solidify as granite and syenite. According to this view the eruptive rocks
had their origin in the upper zones of the earth's crust which once formed the
bed of the ocean.

" This theory presumes that in a higher zone, that is to say, in that which
included the sedimentary deposits of the primoeval ocean, considerably high
temperatures were produced from within the sphere, and explains this as the
result of elevation of the surface of the Earth through the deposition of vast
stratified systems, so that the isothermal planes of temperature were trans-
ferred upwards. In this way, by the filling-up of an ocean of over 10,000ft.
in depth, through the deposition of a mighty complex of stratified rocks, the
temperature of the originally superficial formations was raised to about 100C.

Mechanical (tectonic) Regional- Metamorphism.

" In some places, always however in regions very limited in space, it has been
established as a fact, that the clastic normally-constructed strata, where they
have undergone specially intensified pressure, in the process of mountain-
building, and thus exhibit complicated disturbances of their stratification,
have taken on at the same time a crystalline habit. Observations of that
sort have afforded the tempting opportunity to refer the crystalline schists
generally and collectively to practically similar processes, and the developement
of their crystalline character to the energetic thrust, folding, and pressure,
which originally clastic strata have suffered, together with the heat generated
by the accompanying friction and pressure. Indeed, so far has this idea gone,
that the archaic gneisses and granulites extending through whole regions have
been accounted for as eruptive rocks metamorphosed by pressure into schists
along with the rocks which are subordinately interstratified with them. All
these hypotheses as to such a widespread mechanical metamorphism prevailing
similarly through vast areas and resulting from the pressure concerned in
mountain-building, dispense however with convincing evidence for their data,
and stand in part in direct contradiction to known facts. They are disproved
by the simple fact, that many highly crystalline archaic regions exhibit in the
highest degree a simple little disturbed architectonic structure, while the

APPENDIX ii. 135

neighbouring Silurian and Devonian areas, in spite of powerful folding,
crumpling, overthrusting, crushing, and transversal cleavage have retained
their original structure unchanged, as slates, (Thonschiefer), grauwacke,
sandstone, and ordinary limestone.

HydrocJiemical Regional Metamorphism.

" In contradistinction to these views as to the origin and process of meta-
morphism by plutonic action of heat or by pressure of whole systems of stratified
rocks, the hydrochemical theory of general metamorphism, as it was especially
taught by Bischof, recognizes in the saturation of the rocks with water
through a vast period of time the cause of the metamorphism of rocks on a
large scale, and ascribes to this a substantial alteration and recrystallization of
metamorphosed strata. According to it the process consists in the transference
to depths in the Earth of the materials taken up in the superficial strata of the
Earth's crust, by the chemical activity of water in decomposing and
dissolving mineral substances. This water, holding carbonic acid and oxygen,
penetrates, after its precipitation from the atmosphere, through the rocks near
the surface, where the oxygen it holds is taken up in processes of oxidation and
the carbonic acid is removed from it by the decomposition of certain silicates,
until both gases, after the water has penetrated to greater depths, are used up
entirely and so these reactions must cease. When these waters laden with the
soluble mineral- substances arrive at the more deeply seated strata, they serve
as agents in the alteration of rocks. The alkali and lime -silicates transported
below in this fashion combine with the alumina- and magnesia-silicates already
present there to form compound silicates (e.g. felspar, mica,) which, since this
process goes on exceedingly slowly, separate out in the crystalline form.
Besides single silicates which combine with one another, silica is present in
larger quantities than is required for the resultant double silicates, and so in
this process there is a separation- out of quartz. Thus the hydro-chemical
metamorphism of rocks consists in the introduction of mineral-solutions from
the more superficial zones into the deeper-seated ; further in effecting pari
passu combinations and decompositions between those solutions and the rock-
materials which are saturated with them ; and lastly, in the resultant
formation of new minerals which, on account of the slowness of the process,
crystallize. The result is the complete transformation of the chemical
composition, of the petrographical constitution, and of the structural habit of
the original rock. This process, even when aided by the pressure of the over-
lying strata and by the increase of temperature at depths, requires enormous
time. The direct consequence of this theory is that (e.g.) from one and the
same limestone, according to the nature of the circulating mineral-solutions
and the chemical processes induced thereby, in some places a pyroxene- or
amphibole-rock, in others a garnet- or epidote-bearing rock, in others again, a
quartz- or felspar-rock may be developed.

" A capital objection to this theory of hydro-chemical metamorphism consists
in the fact that it requires an enormous period of time, longer even than that
which has elapsed from the beginning of the palaeozoic ('Silur ') to the present,
for the complete morphological transformation of the rocks through saturation
with water ; since all the formations from the Silurian down to the_ most
recent ore found, where their normal construction is displayed, not yet in the
condition of metamorphosed rocks.* On the contrary, all the palaeozoic
formations, and especially so the Cambrian and Silurian stratified systems,
which follow immediately upon the crystalline schists, contain rolled fragments
both of gneisses and of crystalline schists, which possess exactly the same
structural character as their parent-rock (Muttergestein). The hypothetical
long-continued process of hydrochemical metamorphism of the pre-Cambrian
formations was therefore already completed at the entry of the Earth into the
palaeozoic stage of its developement (in die silurische Periode), and could not

* Compare ii. pp. 16-17 of this work.

136 ROCK-METAMOKPHISM.

have taken the long time demanded for it. Were this really the case all our
lower palaeozoic formations must long ago have been transformed into
crystalline schists and gneisses.

Originally crystalline developement and Diagenesis.*

" Besides the grounds of objection alleged above a few geologists have
perceived a number of other forcible objections, which compel them to hold
that upon the whole such general metamorphism as the earliest sediments have
undergone was of a special character ; nay, more, that their rock-character
which we see in them to day may be primitive and original ; original in the
sense in which this word is used of the shales, conglomerates, sandstones,
marls and limestones. Of these objections, in addition to those alleged above,
the following may be urged :

" (1). Everywhere, where the group of the archaic formations is known,
whether it be India or Scandinavia, in Canada or in Bavaria, J- it is made up of
the same members, possesses the same petrographical structure, producing in
the individual parts the same accessory constituents, presenting in its
petrological variations the same different kinds of rock. Such a complete
resemblance in the petrographical character of the 30,000 metres of strata
comprehended in it, cannot possibly be the product of an alteration and of the
accidents of a process of crushing or saturation with water.

" (2). The variations in the nature of the members of the gneiss- and schist-
series both in the thin layers and in complex stratified masses, are in complete
accord with the differentiation of the strata ; the two things have a mutual
dependence on one another. In this respect the successive changes in the
materials correspond with alterations of conditions which have determined
their stratification. On the other hand, had their structure resulted from a
mechanical or hydro-chemical metamorphism we must have had confused
assemblages of rocks of different character, and not sharply-defined stratifi-
cation planes, often only a few centimetres apart of rocks quite different in
their habit.

" (3). The general parallelism of the arrangement of the mica-laminae and
the hornblende -prisms in the mica- and hornblende-schists, the adhesion of the
mica-lamellae to accessory minerals (e.g. garnet) can be very well explained by
an original developement of this mineral [in the genesis of the rock], not by a
later developement out of solid rock-material.

" (4). If on correct grounds it is accepted as an established fact that the
indi visualization of the felspar-crystals of the porphyries and the trachytes,
and the fracture and separation of the fragments of the orthoclase and
sanidin-crystals had already taken place before the solidification of the
intervening ground-mass, the same inference may be fairly drawn with
reference to the accessory crystalline individuals of hornblende, tourmaline,
garnet, spinell, staurolite, apatite, etc., as they occur in the gneiss- and schist-
formations ; since here also fractured portions of tourmaline-, zirkon-, and
garnet-crystals occur pushed aside from one another and enclosed in the
rock-mass.

" There is moreover no single observed fact in the largest region of the
pre-Silurian (pre-Cambrian) crystalline -schist series in the whole of Germany
(that is to say, in the Ezzgebirge and in the mountains of the East- Bavarian
frontier), which answers to the supposed developement of the phyllites
(Urthonschiefer), the mica-schists, and the gneisses by any later metamorphic

* G. W. Giimbel : Osibayer. Grenzgebirge, 1868, p. 833. See also p. 166.

H. Credner : Gliederung der vorsilur. Format, Nordamerikas (Zeitsch. f. d.
Ges. Nat., 1868. Nos. 11, 12, p. 353; Neues Jahrb. f. Min., 1870, p. 981.

Pfaff : Allg. Geol. ah exacte Wiss., 1873, p. 145.

t According to Mr. Macpherson, the same regularity of succession is seen
in the crystalline series of the Spanish Peninsula. (See Proces- Verbal pour le
19 Septembre, of the Int. Geol. Congress of 1888).

APPENDIX ii. 137

process, in either the plutonic, mechanical, or hydro-chemical sense of the
word. Much more do the constant transitions of the different rock-groups of
these primary mountain-masses along their whole parallel bounding surfaces,
with the identity in kind or close resemblance of their accessory constituents,
of their variations in texture and in the intermingling of their essential
constituents, as they recur similarly in the corresponding strata, plainly show,
that the members of the archaic group of formations are the product of an
universally direct developement (das Produkt allmahlicher direkter Aussehei-
dung.)

" In the regular succession of the gneisses by the mica-schists, of these by
the phyllites, and finally of the last-named by the fossiliferous palaeozoic slates
(Schiefer), we should have, according to the theory which regards the
morphological character of the archaic series of strata as primordial or
original, only to take into account a process of differentiation changing
materially with time, and a difference of external conditions determining the
developement of the separate masses, a gradual diminution of conditions
favourable to simple chemical change,* and an increased action of mechanical
and organic agencies in the formation of rock -material, along with which
(pari passu) a gradual falling-off of the tendency to the separation-out of
crystalline rock-constituents has come about."

[This general conclusion of Dr. Credner's agrees, it will be seen, sub-
stantially with that at which I had previously arrived from my own lines of
investigation, and the agreement is very gratifying to me.]

* That is to say, as the result of 'dissipation of energy.'

BEECUOFT, PRINTER, READING.

ERRATA.

p. 16 (foot note*)

for ' 2 NaSiO 3 ,' read 2 Na 2 Si0 3 .

p 32, line 20

for 'App. ii,' read App. ii, Note G. [The curious mineral referred to
requires further investigation. It contains Al, Ca, and Ba, and a trace
of Fe (as bases), while, along with SiO- 2 , some sulphuric and phosphoric
acids are also present. All these exist in the unfused clastic matrix of
the rock.]

p. 37, line 17

for ' in ' read into.

p. 54

The asterisk has dropped out of the foot-note.

p. 80

(foot-note* ) for ' taken of ' read taken off.

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SOCIAL HABITA TIONSAND PARASITIC
NESTS. With 18 Illustrations. Crown
Svo. 2s. cloth extra, gilt edges.

THE BRANCH BUILDERS. With 28
Illustrations. Crown Svo. 2s. 6d. cloth
extra, gilt edges.

WILD ANIMALS OF THE BIBLE.
With 29 Illustrations. Crown Svo. $s. 6d.
cloth extra, gilt edges.

DOMESTIC ANIMALS OF THE BIBLE.
With 23 Illustrations. Crown Svo. 3.5-. 6d.
cloth extra, gilt edges.

BlRD-LlFE OF THE BlBLE. With 32
Illustrations. Crown Svo. $s. 6d. cloth
extra, gilt edges.

WONDERFUL NESTS. With 30 Illus-
trations. Crown Svo. $s. 6d. cloth extra,
gilt edges.

HOMES UNDER THE GROUND. With
28 Illustrations. Crown Svo. 3*. 6d. cloth
extra, gilt edges.

Wood-Martin. THE LAKE
DWELLINGS OF IRELAND: or Ancient
Lacustrine Habitations of Erin, common-
ly called Crannogs. By W. G. WOOD-
MARTIN, M.R.I.A. Lieut. -Colonel 8th
Brigade North Irish Division, R.A.
With 50 Plates. Royal Svo. 25*.

Wright. HIP DISEASE IN CHILD-
HOOD, with Special Reference to its Treat-
ment by Excision. By G. A. WRIGHT,
B.A. M.B.Oxon. F.R.C.S.Eng. With
48 Original Woodcuts. Svo. los. 6d.

Wylie. HISTORY OF ENGLAN
UNDER HENRY THE FOURTH. By JAMES
HAMILTON WYLIE, M.A. one of Her
Majesty's Inspectors of Schools. (2 vols. )
Vol. I, crown Svo. los. 6d.

Wylie. LABOUR , LEISURE, AND

LUXURY; a Contribution to Present
Practical Political Economy. By
ALEXAN DER W YLI E, of Glasgow. C ro wn
Svo. is.

Youatt. WORKS BY WILLIAM

YOU ATT.

THE HORSE. Revised and enlarged
by W. WATSON, M.R.C.V.S. Svo.

. Woodcuts, ;j. 6d.

THE DOG. Revised and enlarged.
Svo. Woodcuts. 6s.

Younghusband. WORKS BY

FRANCES YOUNGHUSBAND.
THE STORY OF OUR LORD, TOLD
IN SIMPLE LANGUAGE FOR CHILDREN.
With 25 Illustrations on Wood from
Pictures by the Old Masters, and
numerous Ornamental Borders, Initial
Letters, &c. from Longmans' Illustrated
New Testament. Crown Svo. 2s. 6d. cloth
plain ; 3-r. 6d. cloth extra, gilt edges.
THE STORY OF GENESIS. Crown Svo.
2s. 6d.

Zeller. WORKS BY DR. E.

ZELLER.

HISTORY OF ECLECTICISM IN GREEK
PHILOSOPHY. Translated by SARAH
F. ALLEYNE. Crown Svo. icw. 6d.

THE STOICS, EPICUREANS, AND
SCEPTICS. Translated by the Rev. O.
J. REICHEL, M.A. Crown Svo. i$s.

SOCRATES AND THE SOCRATIC
SCHOOLS. Translated by the Rev. O. J.
REICHEL, M.A. Crown Svo. los. 6J.

PLATO AND THE OLDER ACADEMY.
Translated by SARAH F. ALLEYNE and
ALFRED GOODWIN, B.A. Crown Svo.
IBs.

THE PRE-SOCRA TIC SCHOOLS : a His-
tory of Greek Philosophy from the Earliest
Period to the time of Socrates. Trans-
lated by SARAH F. ALLEYNE. 2 vols.
crown Svo. 30^.

OUTLINES OF THE HISTORY OF
GREEK PHILOSOPHY. Translated by
SARAH F. ALLEYNE and EVELYN
ABBOTT. Crown Svo. los. 6d.

GENERAL AND SCIENTIFIC BOOKS PUBLISHED BY LONGMANS & Co.

EPOCHS OF ANCIENT HISTORY.

A. and by C. SANKEY, M.A. 10 volumes,
price 2s. 6d. each.

Edited by the Rev. Sir G. \V. Cox, Bart. M.
fcp. 8vo. with Maps,

THE GRACCHI, MARIUS, AND SULLA. By

A. H. BEESLY, M.A. With 2 Maps.

THE EARLY ROMAN EMPIRE. From the

Assassination of Julius Caesar to the Assassination
of Domitian. By the Rev. W. WOLFE CAPES, M.A.
With 2 Maps.

THR ROMAN EMPIRE OF THE SECOND CEN-

tury, or the Age of the Antonines. By the Rev.
W. WOLFE CAPES, M.A. With 2 Maps.

THE ATHENIAN EMPIRE FROM THE FLIGHT

of Xerxes to the Fall of Athens. By the Rev.
Sir G. W. Cox, Bart. M.A. With 5 Maps.

THE RISE OF THE MACEDONIAN EMPIRE.

By ARTHUR M. CURTEIS, M.A. With 8 Maps.

THE GREEKS AND THE PERSIANS. By the

Rev. Sir G. W. Cox, Bart. M.A. With 4 Maps.

ROME TO ITS CAPTURE BY THE GAULS.

By WILHELM IHNE. With a Map.

THE ROMAN TRIUMVIRATES. By the Very

Rev. CHARLES MERIVALE, D.D. Dean of Ely.
With a Map.

THE SPARTAN AND THEBAN SUPREMACIES.

By CHARLES SANKEY, M.A. With 5 Maps.

ROME AND CARTHAGE, THE PUNIC WARS.

By R. BOSWORTH SMITH, M.A. With o Maps
and Plans.

EPOCHS OF MODERN HISTORY.

Edited by C. COLBECK, M.A. 19 volumes,
THE BEGINNING OF THE MIDDLE AGES.

By the Very Rev. RICHARD WILLIAM CHURCH,
M.A. &c. Dean of St. Paul's. With 3 Maps.

THE NORMANS IN EUROPE. By Rev. A.

H. JOHNSON, M.A. With 3 Maps.

THE CRUSADES. By the Rev. Sir G. W.

Cox, Bart. M.A. With a Map.

THE EARLY PLANTAGENETS. By the

Right Rev. W. STUBBS, D.D. Bishop of Oxford.
With 2 Maps.

EDWARD THE THIRD. By the Rev. W.

WARBURTON, M.A. With 3 Maps.

THE HOUSES OF LANCASTER AND YORK;

ivith the Conquest and Loss of France. By
JAMES GAIRDNER. With 5 Maps.

THE EARLY TUDORS. By the Rev. C. E.

MOBERLY, M.A.

THE ERA OF THE PROTESTANT REVOLU-

tion. By F. SEEBOHM. With 4 Maps.

THE AGE OF ELIZABETH. By the Rev. M.

CREIGHTON, M.A. LL.D. With 5 Maps.

fcp. Svo. with Maps, price 2s. 6d. each.
THE FIRST Two STUARTS AND THE PURI-

tan Revolution. 1603-1660. By SAMUEL RAWSON
GARDINER. With 4 Maps.

THE THIRTY YEARS' WAR, 1618-1648. By

SAMUEL RAWSON GARDINER. With a Map.

THE ENGLISH RESTORATION AND Louis

XIV. 1648-1678. By OSMUND AIRY.

THE FALL OF THE STUARTS ; AND WESTERN

Europe from 1678 to 1697. By the Rev. EDWARD
HALE, M.A. With n Maps and Plans.

THE AGE OF ANNE. By E. E. MORRIS,

M.A. With 7 Maps and Plans.

THE EARLY HANOVERIANS. By E. E.

MORRIS, M.A. With 9 Maps and Plans.

FREDERICK THE GREAT AND THE SEVEN

Years' War. By F. W. LONGMAN. With 2 Maps.

THE WAR OF AMERICAN INDEPENDENCE,

1775-1783. By J. M. LUDLOW. With 4 Maps.

THE FRENCH REVOULTION, 1789-1795. By

Mrs. S. R. GARDINER. With 7 Maps.

THE EPOCH OF REFORM, 1830-1850. By
JUSTIN MCCARTHY, M.P.

EPOCHS OF CHURCH HISTORY.

Edited by the Rev. MANDELL CREIGHTON.
THE ENGLISH CHURCH IN OTHER LANDS.

By the Rev. H. W. TUCKER.

THE HISTORY OF THE REFORMATION IN

England. By the Rev. GEORGE G. PERRY.

THE CHURCH OF THE EARLY FATHERS.

By ALFRED PLUMMER, D.D.

THE EVANGELICAL REVIVAL IN THE

Eighteenth Century. By the Rev. J. H. OVER-
TON.

A HISTORY OF THE UNIVERSITY OF OXFORD.

By the Hon. G. C. BRODRICK, D.C.L.

A HISTORY OF THE UNIVERSITY OF CAM-

bridge. By J. BASS MULLINGER, M.A.

THE ENGLISH CHURCH IN THE MIDDLE

Ages. By Rev. W. HUNT, M.A.

ARIAN CONTROVERSY. By II. M.

GWATKIN, M.A.

16 volumes, fcp. Svo. price 2s. 6d. each.
THE COUNTER-REFORMATION. By A. W.

WARD.

THE CHURCH AND THE ROMAN EMPIRE.

By the Rev. A. CARR.

THE CHURCH AND THE PURITANS, 15/0-

1660. By HENRY OFFLEY WAKEMAN.

THE CHURCH AND THE EASTERN EMPIRE.

By the Rev. H. F. TOZER.

HlLDEBRAND AND HlS TlMES. By the
Rev. W. R. W. STEPHENS.

THE POPES AND THE HOHENSTAUFEN.

By UGO BALZANI.

THE GERMAN REFORMATION. By Prof.

MANDELL CKEIGHTON.

WYCLIFFE AND EARLY MOVEMENTS FOR

Reform. By R. LANE POOLE.

luare, London.

IM\ KRSITY OF CALIFORNIA LIBRARY

THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW

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Full text of "Chemical and physical studies in the metamorphism of rocks based on a thesis (with appendices) written for the doctorate in science in the University of London"

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